Re: Regrading grabs in Aikido...
One answer I haven't seen yet, is that you train with ushiro ryoutetori for the same reason you train morotetori. Not because it's necessarily a likely situation in combat (none of the kihon waza are really intended as realistic self-defense scenarios), but because it trains in the essential principles of the art. In this case, using kokyuu-ryoku (rather than muscle strength), keeping one's power centered, understanding maai, etc. In theory, if you can properly do a technique from a static ushiro ryoutetori, you can do it from a flowing ushiro katatori, kubishime, etc.