I think that's still a little too restrictive. Sometimes you really do want multiple groups of windows that may belong to the same (think multiple browser windows each with multiple tabs) or different applications (e.g. grouped by task). It's not hard to see how the application marketplace leads to every app doing everything including managing all the things it does, but it's not good for the user.

Custom groupings is a nice feature too, but that feature can live happily alongside app groups. In fact I think the two would compliment each other nicely.