Yeah, being able to aim independently of moving works in Binding of Isaac because you have only one weapon. It wouldn't work so well if you have 4, so I can understand completely why you wouldn't want to go with it this way.
Sure you could allow independent aiming if you had only switched between active weapons, but I've found that really discourages you from using variety, as you just keep one active and never switch. In cave story for instance, I would stick to one weapon and never switch mid-fight because its too annoying to do, if they were bound to different keys I would.
I'm curious what sort of different mage classes we'll see. It would be cool if there were some completely different mechanics behind the classes. As in one that has a cave story mechanic of charging up to better weapons the more enemies you kill without getting hit. Or some classes that do self-damage in exchange for big hits. I'm hoping to see not just different types of spells, but different ways of playing the game completely!
Especially as this will be multiplayer, we can have some real specialized classes (along with general ones of course) lets get for instance a crowd control class (maybe it does less damage but has a paralysis shot, or can make barriers). A damage over time class. A high risk/high reward class. One with more AoE stuff. In short, don't be afraid to go crazy on the classes as that'll add a huge amount to the replay value.