What a great thread with very helpful healers everywhere. UOdhn already gave a great list and reasons for the cross class skills, I agree with him on that list. I would like to expand on the Triage response by subteraneanbird (oh gosh, what a hard name to type! ha ha).

I was a DRK and RDM in FFXI and when I started with 1.0 I started as a WAR. When 2.0 released, I decided to do something different and did my story quests as a CNJ/WHM and ended up loving it so much it became my main now (more so than WAR). Everything was dandy until I got to Suken Temple of Quan and I had a tough time healing. I had to learn the this the hard way: heal yourself first before others.

Triage:
As a healer, like subteraneanbird said, is to keep the group alive, but not necessary everyone alive. Tank in this game, for the most part, can take quite a few hits before going down. If you get hit by an AoE along with the group, your first priority to heal YOURSELF.