Some small rebuttals/points.
- Personnel - This is the magic source of the Burning Blade Expedition's sole Caster-type Hero. Grok is taking steps towards getting his Shaman/Caster on, but for the moment Keldran is it. He also represents the best route the thread has right now of understanding and countering any actions from the Old Gods, Void, or general Shadow users. He also represents the chance for the Burning Blade to provide knowledge to the Crusade and their allies they would have no other easy method of accessing. His way of channeling the Shadow is likely unique, given the odd history of the Necrolytes. It stands to reason the the Shadow Priests who come at it through the Light, such as the Forgotten Shadow cult the Forsaken seem to have, would come at it different way. Finally, investing actions here now gives the potential for more home grown casters later down the line. The thread has had some suggestions of asking the Crusade for Light-wielding resources, I believe this would compliment that. The sooner Grok knows what he is about, the sooner the thread will be comfortable utilizing him.
Not his sole magic source he's a fel-shadow user and not especially great at either of them, if he was he would already be long dead.
I disagree with the assessment that we need shadow magic to counter shadow magic, fire and the like works very well and using shadow on the old gods traditionally backfires.
Incorrect, they've got access to the works of Nataline Seline
Natalie Seline, in fact several of the crusade leaders likely knew her personally, in fact there's a cache of her teachings in Lorderan right now, which is where the cult of the Forgotten Shadow comes from there are more in Dalaran's ruins and scattered all over the place to ensure they wouldn't be lost.
Sorta. The alliance tradition got started because Seline researched the fel through observing necrolytes. Furthermore
different in this case does not mean better to restate once again, Keldran was only taught because he was taught to a certain level and no further, while lacking the talent necessary to progress much further on his own otherwise he'd have been executed or brought with Gul'dan/Cho'gall when they did their betrayal.
Furthermore what he was taught was filtered to him through Gul'dan. I'd argue there'd be legs to stand on if it had been Ner'zul whose shadow magics came from the Shadowmoon's study of a Na'ru turning into a Void God, but Gul'dan's is quite literally from the legion, right down to the name which he lifted wholesale from an Eredar named Satiel.
Thus we're left in an ironic situation that by learning about the scourge we're probably learning about a more unique branch of necromancy than the one Keldran was actually taught and the most popularly learned ones since discounting any shadow lands stuff the Scourge are a combo of Legion/shadow moon methods.
And I'd say hold your horses since down the line is very far down the line and again I reject the premise that in order to know what Shadow does he needs to be able to do it himself.
- Plot - With the Paladin preaching, and the upcoming summit, it seems like this arc will have good chances to confront some specters of the First and Second Orc/Human war. Keldran is as intimately tied up in this as Neeru is, given that they both survived the purges between the two. Learning more about his rapid aging, and his experiences with the wars would provide another interesting viewpoint on this all. Grok-wise, his current naivety/general ignorance of magical matters makes this useful as well. The fluff text for the Light action involves him asking about the differences between a priest and a shaman. Having that next to a discussion about what sort of church Gul'dan wrought opens up more chances to sort these things out in a constructive way. Find the lines between horrid things to outlaw, useful things to grudgingly use, and actual good things to carry forward involves looking and sorting. Finally, by knowing more about Shadow Magic Grok and the thread will learn more about the similarities/ differences between it and the Necromancy which scars Lordaeron.
It also makes him potentially vulnerable to manipulations from these people...also Church of Gul'dan?
Now there's a nightmare scenario.
As for that last bit I don't see that as especially useful. The line between what is and what isn't acceptable changes a great deal with the situation, but as I've gone to pains to emphasise when the entry level of the discipline involves soul mutilation that should get an automatic hard no.
As far as I'm concerned the answer is to get strong enough that we don't have to use things like the giant soul torture murder dog in the first place.
- Potential - A very meta reason, but this is very much a start as one means to go on. WOG says that even the Black Dragons are not beyond saving, so its worth it in my humble opinion to invest actions learning about the forces normally used by mainly by antagonists. Keldran is apparently strong enough in magic to be considered important in a mainly Warlock clan. That shows he is worth learning from, even if Grok never touches the Shadow for himself. But given his generally high magic skill and the lack of current build commitment, taking a wide range of learning actions as opposed to mono-focusing seems a worthwhile path.
My argument would be that two wrongs do not make a right. Instead of using shadow magic to try and purge the influence of the literal gods of that magic instead opting for an alternative that is supposed to be its natural weakness might be a better idea. That and its usually used by the antagonists for a reason, lets poke it when it won't eat us.
As for important in a mainly warlock clan, well aside from the irony of what his NPC counterpart does in game

where did you get that he was important and not just the only person they had for the job who could be spared.
We're already trying to do two separate magic traditions at the same time, that's the opposite of monofocusing, furthermore as has been banged on about repeatedly, wide range means we also get nothing done.
Your aspirations don't mean anything if they don't actually go anywhere.
As Grok becomes more skilled in magic his ability to learn other things isn't about to go down, it goes up especially since its that lack of knowledge which has been holding him back. So no taking a wide array of options has the same problem that taking a wide array of options does in real life, without mitigating circumstances usually you end up doing a bunch of them poorly instead of a few well.
- Personal - Having tried to remove my own opinion for the last few points, this is all my bias. I like the idea of the taking advantage of the Orc's lack of living magical traditions to make new ones. I prefer a rounded approach to things before committing to any build. I personally think that establishing a trend of taking multiple different Magic training actions will lead to a better story, as I would like to one day vote for studying Arcane/Fel side by side too. Nature/Necromacy is a bit harder to think how to pull off, but its still cool!
And I would agree with you, I would also like to point out that it has been less than a year in universe for Grok in which time he has successfully unlocked then lost one magic tradition and is making progress on unlocking (and when we invariably fuck up again I'm sure loosing) another.
At the same time we are already committed to a build its called being a blade master, otherwise there is nothing that's going to lock us out of any tradition aside from Grok dying* which is a genuine risk.
*I was going to say some kind of reforging, but given that they managed to light forge a freaking Nathrazim I think that's not a risk either chief.
As you say I'm sure you won't be torn up if the shadow thing doesn't win, but my view remains the same, our time crunch is in "can we get strong enough that Jub does not gank us...again" not "does Grok have enough talent/time to theoretically learn this magic stuff." He clearly has the talent for everything not involving wind, and has the curiosity to try, but we've got to temper that with reasonable expectations and practicality
Currently going down the light chain is likely to help get Grok in a better mental state, which in turn will help get him his Shamanism back as well as prepare him for Jub by giving him something which really screws over the undead, which means he can get access to things like say the library of the Kiran Tor. Lots of doors open to war heroes who've fought bravely against something like the scourge.
To also advocate a bit for why the light would be good on a more plot level, this would be an Orc the heir to a very infamous warlock clan no less, learning the light which is a very important cultural thing to the humans and something that has never happened before. The special snowflake in me is interested and also thinks that as far as making an impression on the alliance goes we can't really get much better than it if we pull it off.