I don't know too many teachers who can actually use the finer functionality of MS office. A reluctance to switch must have another origin.
The one place where I've had trouble with OO occurs when I sometimes teach using prepared presentations. I can modify them in OO, but if they have a bunch of A/V goodies baked in I'll use ppt player to present so that everything works easily. I usually run a show like that out of Windows, too.