I'm wondering if someone could shed some light on this question for me... Why does ext GWT (and MyGWT before it) define this DataCallback interface, rather than leveraging the "standard" AsyncCallback defined in GWT itself?

I can see the obvious differences...only have to implement 1 method instead of 2, and check a flag for success or failure. I'm looking more fundamentally...what problem does this new interface solve, that can't be solved (or is kinda hard to solve) with just an AsyncCallback?

Please don't take this question the wrong way... I'm just trying to wrap my head around some more of this library, and want to make sure I understand as best I can how to leverage the library.