I have never figured out why anyone would want this. Maybe I'm just a newbie, because I didn't start using X until fairly late in the 80s. Maybe if I had been using it from the very beginning, focus-follows-mouse would make more sense.
Mostly so that you can type into a shell window without having to bring it to the front. At least that's what I miss the most. Not a deal breaker, but once you get used to it, it sucks to lose it.