I don't disagree with you and I'd love to see some more casters but the question is what kind of caster could they add? We have the DoT based caster that builds to a backloaded burst and a spell based caster that's more about front loaded burst. We don't really have a support/CC/debuffer role that's available or necessary for PVE. Best I can think of is a caster that's more between the two that has a little to no burst and is more about sustained dps with a possible damage/speed ramp up similar to a mnk.