It goes through spirit like nobody's business, and most groups are based around efficient farming, so having a dedicated healer would be pretty crappy. It's basically just an oh-shit button for the Monk.
Paladins were also able to heal others in D2 using Holy Bolt, and I never, ever saw anyone do that. I don't know, if D3 is anything like D2, healing will be incredibly less efficient than just killing.
My Gaming Setup | WoW Holy Paladin (retired) "This is not a dress. This is a sacred robe of the ancient psychedelic monks."
Healing's just there to make the Monk more unique and make them function a bit differently, that's about it. No need for a dedicated healer in a Diablo game, you just kill stuff. It's a nice little bonus, but nothing more.