I was thinking about getting a fish for my desk at work...figured a betta would be good, but I thought the same thing you did...you never see them MOVE.
OK long story short I was telling one of my friends that and he used to have bettas, and absolutely loved them! He said that in the store they
A: dont have much room to move and
B: the water is usually cooler than they like it.
He said that Betas move a lot more when the water is warmer, not just room temperature like most betas are kept (because their little tiny homes can't accomodate heaters!)
After talking with him about them I decided that one day I MIGHT get one...but I'm still not sure