I think that another part of the reason Casters have the charge times is to, well, make them feel like spellcasters. Ranged and Melee DPS classes don't have cast times, at least now they don't *glares at HW Wanderer's Minuet*. When you play a Spellcaster, you want to feel that glorious rush of power as a spell goes off at just the right moment, or the terror of an enemy approaching while you desperately make your incantations and such. Having casting bars helps bring that feel into Caster classes and helps their identity compared to other roles.