"Easily bypassed OR unimportant."
I don't really care about that. There are already tons of inconsistencies - just try running a GNOME app next to a KDE one and try to count up all the different inconsistencies you can find. Adding the window decorations to that list seems like a pretty unimportant addition to me.
And within GTK/Qt apps, i think the toolkit would provide a pretty good job at keeping things consistent. There would obviously be some issues that would pop up, but again - i don't really care.
Other people apparently do, so I'm not necessarily against KWin providing SSDs. I just personally don't find it important.

