Drive space. I find this reason a bit silly, but I KNOW there are people out there who take the time to strip other languages out of iLife programs to save a few MB, so someone out there will like it for this reason.
Just reformatted my brand new MacBook. By eliminating all the unneeded extras like fonts, languages, and printer drivers (3.1 GB of printer drivers! No wonder things "just work" in Mac OSX!), X11, and iDVD/Garage band extra content, you can cut the 15 GB Mac os X install down to 6 GB.
That's a lot of stuff to clear out!