I think spell casting is just fine as it is. I know several BLMs and SMNs that perform very well and they love it. I actually love playing SMN, but I'm usually WHM. This isn't WoW...you have to think and not just stand there face-rolling til HP goes to 0. Got a melee on you? Surecast > Blizz2, get a some distance, sleep it, rape it. Bard on you? Run behind the neareast wall/rock, Scathe and kite it to death. As you said, these methods don't always work because of CC immunity abilities...but casters have a big advantage because you can Swiftcast every minute, Surecast every 30 seconds while Fetter Ward can only be used every 2 minutes. Tempered Will has an even longer recast.