Agreed. I'm going to guess that Februs is right concerning the damage threshold, or else it has to be a specific type of attack to interrupt spells. I'm rarely interrupted by casters, and not as often by MNK...but DRG, DRK and WAR have a field day interrupting my casting.
As for people that complain about healers not staying stunned, there are many ways to escape a stun. I use Purify all the time. It even works on Holmgang. I can also use Equinomity (sp?) to be sure that my spell won't be interrupted, or use Surecast to achieve the same result. I have seen it fail at times - I'm not sure why - but it typically works out well. And then there's Swiftcast, which is enough to throw out an Aspected Benefic or Essential Dignity to turn the tide of battle, or Lightspeed.
In all, as AST, I have at least four moves that will allow me to work around or completely negate stuns and interrupts pretty effectively. That's not factoring in the most basic mechanic:
I can sprint out-of-reach all day long. I don't need TP. Melee-users do.