I really can't see how those but washers (the likes you see in Japan etc) really work?!
Surely it's like trying to wash your car with just a pressure washer? Yeh it might get a bit cleaner, but you're only really going to properly clean it with a physical contact. Ie with a mitt etc
If you got poo on your hand would you simply wipe it off with tissue or would you wash your hands? You still do both with a bidet.