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.
Economists can certainly disappoint you. One said that the economy would turn up by the last quarter. Well, I'm down to mine and it hasn't. -- Robert Orben