With the round back all the sound goes out front. There is very little vibration coming back towards you, therefore you don't "feel" the sound coming into your body as you do with wooden backed guitars. As for why do Gibson and Martin not use round backs - origionaly the round back was under copywrite, so even if they'd wanted to they couldn't have. Anyway when you've spent so long establishing your own style your not going to change just 'cause a newcommer comes along with something new.
It really is hourses for courses. The Ovations are great stage guitars especially when playing in a band, but you need good moniters to hear yourself. As I said before I've loved them for years, but my current favourite is a Tacoma with a traditional back. Still got my Ovies though.
One last thought on shape, both Danny Kyle and Hamish Imlach borrowed mine, and although there was only a single point of contact between them and the guitar, they got on all right.
love, john.