So the speakers are isolated from the wall structure… in theory. It seems like you should still get some resonance off the front wall. It also seems that, by not tight fitting the speakers to the wall, you would be creating a port which would alter your frequency response curve.
Not to mention… What if you decide to move them a few degrees or inches? 
SOL
I guess I just see it done WRONG more often than not. For example…
This guy is just building boxes or shelves into his wall and calling it soffit mounting. Seems like a bad idea…