Why not use AI servo all the time? (50D)

Is there any reason why you shouldn't have "AI servo" turned on all the time (as opposed to switching between it and "One shot" modes)?

I mean, for subjects NOT moving (portraits etc.) or moving significantly there shouldn't be any reason for the camera to re-focus. Having used "One shot" all the time I recently gave "AI servo" a go in order to better shoot (bad choice of word) my 2 year old daughter who won't stand still

(I've read that "AI focus" should best be forgotten as it doesn't work very well).