Individual applications may still switch to gtk3, in which case those applications will experience the breakages. But that can't be helped-- it's up to the individual application developer. In the same way, you can fire up a KDE application in Xfce and stumble across various KDE bugs.
I have no idea what will happen in the long term with regard to Xfce and gtk. Last I checked, there were no developers working on any version of gtk full-time anywhere. So maybe it would be practical for Xfce to fork gtk 2.x. Or maybe gtk 3 will settle down and they'll switch. We'll see.
P.S. I use Xfce myself. I think it's the best desktop out there today for Linux.