sbjaerum: Why can't sensor shift IS be used during video recording?
"Are there other reasons?"
One very big one is noise - you don't want IS sound on the soundtrack.
Also it is not really needed, at a frame rate of 60/s the exposure time for each frame must be less than 1/60th and so will have very little motion blur anyway. What is needed is to align the frames (remove the movement between frames) and that is best done in software.