I tried using i3 for a while and loved it, but there were various, for lack of a better word, "desktop integration magic" things that I could only get to work reliably running in gnome or unity.

Thinks like hot plugging thunderbolt and having audio work perfectly, monitors readjust automatically, and variable dpi support. Actually so far ubuntu's been the only distro that makes it work well. This is the only reason I use unity.