stowe, I think it could be excellent practice, if you practice the right things. For example, if you practice using the core muscles as the overwhelming source of power in your swing and practice a dead hand feeling in your hand and forearm, the practice can be really good. If you are just practicing compensating moves for not having both hands/arms, it will only make things worse.

You may want to find a copy of Search for the Perfect Swing by Cochran and Stobbs. They talk a lot about a model with one arm, even go so far to talk a fair amount about two one-armed golfers' swings. Adding a second arm does have some advantages, particularly in terms of support and stability, but one can accomplish a very powerful and accurate swing with just one arm -- the key is a core driven swing.