OK, here’s one good solid rational usability reason the MS Office Assistant can be annoying: the default location covers the lower, right corner of the window. This sucks in Windows where, because windows maximize to fill the whole screen, there are about 6 interface widgets that are blocked by the Assistant (scroll up, down, window size handle, etc.).
The Assistant does have an option called “move when in the way” but it doesn’t move as far as I can tell. It DOES remember its last location, so if you put it in a good spot it will go back there next time. But people prolly just get frustrated the first time and turn it off.