Maybe its time for Vanguard to spin out to its own Universal tree...its not like S&B fighting is super OP anyway, and that'd mesh well with other builds like EK and Swashbuckler too, as well as give a melee boost for classes like Cleric and Warlock that have lackluster native support. Just make sure it gets +1/+1 crit baked in while holding a shield.
Though I dont know what you'd replace it with for Fighter that isnt just "Kensei II"
First off, just because an oversight is old doesnt mean its not still an oversight....pleeeeenty of examples of imbalanced mechanics that were left for years before being addressed
Arcane Warrior is meant to support a hybrid playstyle, where you're actively attacking and casting. What's wrong about Aura triggering it is it's entirely passive - I dont think any other source of persistent/passive damage will trigger it. That's what makes it different...you arent required to stop attacking (and therefore lose a round of DPS) and actually cast a spell to build a stack.
That's why, to use your example, casters dont try to whack mobs with their scepters to build stacks of Spellpower. Its not worth the lost round of DPS. That's also why melees dont, say, splash Fatesinger and spam the Epic Strike to build stack, even though you can stay pure that way.
If you want to splash /2 Lock and spam Burst to build stacks, I'm OK with that. If you want to splash /1 Wiz and spam MM SLAs, I'm fine with that too. Or use any Racial SLA. I just think that it should require 1 button press for every 1 stack that's built.