Re: Ground Defense
The goal is to not be on the ground. Aikido is a standing jujitsu art. It's main assumption came from aikijujitsu , where the assumption was that you are a trained Budoka, that has lost his sword, defending against Budoka with swords. The goal is to not get on the ground in a traditional sense. (other than suwazi, which is technically standing anyways.)
The assumption in the classical sense is that your attackers are trained in Budo, and will react like one who is trained in Budo. That's why Uke traditionally reacts the way he does.