Unless it takes a long time (in which case, it's not worth it, as Lex decided) to get even simple magic, any of it would help. For Magic is Supermans greatest weakness that doesn't begin with a K, and since Kryptonite alone apparently doesn't cut it in taking down the Blue Boyscout, might as well ad some magic on top. But he doesn't, and thus, there has to be a reason.
Even simple magic is extremely useful for a wide variety of uses. After all, if magic wasn't useful, there'd be no low level mages, they wouldn't be interesting to write a story about.
Just because there hasn't been talk of a talent for magic, doesn't mean that such a thing isn't a thing. They never said what are the conditions to someone learning magic, so we have to "make shit up". You have covered that there are magic users who aren't Homo Magi, but that doesn't mean that they don't got something special to them. In Marvel, not all Supers are Mutants, but not everyone can be Super. Almost all Supers were special in some way, even if they weren't part of some special race of Supers. Ras in particular is super old, he has had plenty of time to learn the magic the hard slow way even if he personally has no talent. That means that anyone CAN do magic, but would have to dedicate years/decades to achieve what the naturally talented can do in days. He has had plenty of time to hone his craft, he's exceptional, and thus an exception, not the rule.
Those idiots, there are two possible explanations.
1) They got magic. They are one of those who do got a magical talent, and presumably magical souls are more tasty than mundane souls.
2) They didn't really do much magic, it was the Demon that did all the heavy lifting. They drew the runes and did all the prep work the Demon needed, which allowed him to be summoned. After all, Constantine is talking as if the Kids were tricked into this, the Demon explicitly wanted this, which presumably isn't the case in normal demon summonings.
