That being said casters are so much simpler (espically aoe which dont need facing) and with the really fast mana regen in this game why mellee?
Well, there are several reasons. After a bit practice it isn't so hard to position your melee alts in front of the mobs, so playing a group with some casters and some melees isn't worse than playing a full caster group.

I play with one window maximized and the others minimized. As far as I know there is no way to send mouse clicks to minimized windows so I can't use a lot of the mage and warlock AoE spells, like Blizzard and Rain of Fire. But my warrior, rogue and retri paladin can easily chop the mobs to pieces with Bladestorm, Killing Spree and Divine Storm.