"Casters are overpowered" mainly applies to SMN, who can kill without ever casting-- so the point is moot-- and BLM/WHM sleep.

I agree that SOMETHING should be changed. As it stands, only auto-attacks interrupt casting. This mainly becomes a problem when you are ganged up on. So rather than reduce the % chance to interrupt, there should be something to allow casters to survive (or, attempt to survive) being hit by all three opposing dps at once.