FormerlyHemlock
Hero
I don't think a distinction between crunch and fluff is particularly helpful as there is no clear line of demarcation in this "rules as guidelines" edition. *snip* So, I think you're correct in not viewing this type of language as restrictive. But I think its more helpful when interpreting the rules to view these instances as defining certain actions as typical of a hostile creature, rather than to simply disregard the language as "not crunch".
Okay. "Crunch" vs. "fluff" isn't a distinction I make at the table anyway; it's just language which is occasionally useful when discussing things on the Internet. The key point is that I have no problem allowing people to make melee weapon attacks or opportunity attacks on friendly creatures, so for me Mistwell's quandary doesn't exist. But that also means that Warcaster is fairly unique in allowing you to convert full action spells to reaction spells.
I think what I'm likely to do if someone expresses an interest in Warcaster is to say one of the following:
Option #1: Warcaster allows you to cast a spell with an attack roll in place of making an opportunity attack. The game logic in this case is that "spells with an attack roll spend most of their action lining up the attack, in the same way that shooting a crossbow spends most of its action lining up the shot. Pulling the trigger is a trivial effort, and Warcaster just trains you to line up the shot and pull that spell's "trigger" instinctively when a retreating enemy gives you the opening." In short, attack roll spells are physically quicker and easier to cast.
Option #2: When you gain this feat, choose a specific spell with a casting time of 1 action or 1 bonus action and a circumstance under which you cast this spell instinctively. E.g. "When a hostile enemy retreats from melee, I can automatically cast Hold Monster." This is both stronger and weaker than RAW Warcaster, but it is way easier to adjudicate: the game physics explanation is that you've practiced that specific spell to the point where you can cast it as a reaction.
I like option #2 better.