As someone who frequently plays both sides of the coin, I'm iffy on this.

As a healer, getting interrupted is a chore. Playing as a WHM/SCH isn't about how well you can heal, it's how long you can outlast the other team's healer before you eventually cave and hit the ground. Your abilities only prolong the inevitable. If it's getting to the point where you need to use Attunement as your last ditch effort, then it's pretty much game over. It's really more so about sprinting about, swift casting Medica II with divine seal popped and running and trying to get monks and ninjas off my back before I inevitably faceplant.

On the flip side, think about uninterrupted heals. As a DPS, nothing is more irritating than trying to get a healer down only to have him/her just bring their health back up with one cast of Cure II. It really becomes exhausting and just not fun to play as.

Same goes for BLM/SMN. Think about it. When I PLAY BLM, of course I get irritated when I can't cast uninterrupted. It makes me feel useless because I'm not doling out heavy hits like I could be.

But on the flip side, when somebody else is playing BLM uninterrupted? Holy crap, you'd better watch out. Constant 700+ damage every two seconds with several cool downs to down you in a few seconds.

It's a give and take system. You cannot have BLMs/SCHs uninterrupted because then there burst damage becomes incredibly overpowered. On the flipside, being unable to cast ANY spell without swift or sure cast is a pain. But that's where sleep and other abilities come in.