WHile I do agree with the benefits you've provided, I disagree on the value it has compared to just upgrading Snorri. In part because the HG is relatively squishy due to lack of Super Regen and because our main consideration is action economy as we've actively designed the HG to provide and cover for areas that we don't handle.
Like @Cecylene said we use our HG a lot in areas around us without actively following us into combat so we can achieve multiple objectives at the same time. It's unlikely in that use case that a AOE buff around us is effective .
But let's assume that we want our HG to fight alongside us and we don't get LEFT so we need to research and recreate the effect ourselves. How many and which projects do we sacrifice to enable that ability? Do we sacrifice a few turns of our Space and Repair research, or our super Killy Axe, or .....
Additionally for our HG to reliably fight alongside us, they need to be able to keep up with our new doctrine of Airdrop Snorri. Which is another action sink.
THat is not to say it is without merit, but the HG is competing fiercely against other things like Force Multiplying Snorri or recreating/improving anti daemon runes.
So the Hearthguard being squishy is a direct result of us not putting very much effort into them being not-squishy. Like, my point is that using "they are squishy" as justification to not build something that would make them not squishy is feeding into the same logic that got us here in the first place.
And, stepping aside from the "fight with Snorri" idea entirely for a moment, we don't actually want them to be squishy for another pretty significant reason. Missions are getting harder, which means they are more likely to die. And they are going to keep getting harder. The war (any war really) is also going to impact them, regardless of if they fight with Snorri in the storm or not.
And we don't need Left to fix this.
To fix this for the Hearthguard takes some compression, conversion, and building one item - that item also lets them survive in the storm by using a combo involving Banner Mountainsoul. That was tbh, the main design goal, and because of the regeneration and durability involved it also fixes this squishiness issue.
Going back to the fight with Snorri thing, do I want them to fight with him after we build a storm-survival banner? No. But that is also a solvable problem.
And the convenient thing is that we don't have to distract Snorri because we can just use Karstah. My plan next turn for example has us building the Azrilzhufgotten Banner we just designed very recently using Karstah, and that'll be a very nice boost for the Hearthguard.
They also do not need to keep up with Airdrop Snorri, because like other people have mentioned there's other things for them to do where upgrades will help them significantly, or for moments in a field battle where airdropping him is not viable. Or we decide to take other options of Snorri deployment and here's the thing - we've done it a few times, but its not an actual doctrine that we will use Every Time.
