I guess I have the same question as you--where are these people attacking structure, or disregarding it? In any literary department in which I've worked, faulty structure or poor storytelling are pretty strong marks against a script. I've never heard a solid structure attacked, except when that's all there is. If the structure is strong but the characters aren't interesting, or the plot, while well told, isn't credible, or the play has nothing interesting or true to say, then those are the problems. The structure might get faulted, but only because nothing else works. I may be wrong, but I'd love to see who is actually attacking or disdaining story.
I think Zev hits it on the head. Part of the story here lies in the kinds of reviews that Ms. Rebeck's plays get, Mauritius, for instance, has been called both "Mamet-lite" (Trib) and "Mamet unplugged" (TOC), why is that? My guess is that this really does stem from the play's suspenseful structure. The plot takes center stage in a way which I do suppose is unusual today in theater. We talk about plays really being about characters, we sometimes talk about Idea Plays (as though that means something). Really though, the crisis is that attending the theater has become enough of a rarefied experience that Entertainment as such seems something to sniffle at. Ms. Rebeck's choice to situate this snobbishness in plot says more about her own writing than about the broader context of this problem.
Zev's comment that plot is all well and good as long as it is not all there is, is important here because I think that praise and disdain is a little more cyclical. A lot of good plot makes it easier to miss finer points which makes it easy to dismiss writers as "lite" or unsubstantial. Fairly or unfairly.