Personally I can cast 1234, shift+1234, qe, shift+qe. Thats 12 spells. If I ever have to hit T and manually do stuff, it takes me forever, because the cursor blends in too well with the game. This is fine for buffing and metamagics and clickes because I can take all the time prebattle to do it.
So all those debuffs would never take up main slots for me which means I'd be standing still searching my bars for the right spell to click, hoping the spell actually casts (servers are POS) and then hunting for the next one hoping my target hasn't moved and the spell fails to cast because I wasn't facing it.
OR I can nuke things with my camera locked to the mouse and never miss apart from random rolls and stuff (which is dumb when a fireball explodes in a baddy's face and does nothing but I guess its for D&D flavor)
Do not neglect the mechanical aspect of your spell choices either.