I love my PLD, but this mechanic really takes a huge chunk of usefulness and fun out of stunning/interrupting a cast.

In order to have a shot at doing my job (preventing/mitigating damage), I have to stop doing my -most- important job: generating enmity, because I might get stuck in the global cooldown, helplessly spamming my Shield Bash as the enemy casts. Shield Bash is a great idea, but the GCD renders it frequently useless and impotent.

Forcing a tank to stop all actions and wait for a cast is not only inefficient, but also takes skill, reaction time and situational awareness out of the equation. It's just bad design. I hope to see this change in the future.