Mp5  is great but as a certain druid has said its about down time to get benefit so spam healing hurts if you are heavy Mp5 (as i once was).  Why I changed was simple if a fight is not mega long  ( raid boss like)  Mp5 is not used very often or at all.  Plus to healing is what i switched to for now all of what Shach said was spot on but one thing that also is supa huge is (at least till expansion they say) is rank lowering your heals its what i do its huge  instead of casting a rank 7 spell i cast a rank 4 it costs less that half the mana and i still get HUGE heals because of my plus to healing.  It's hard to get used to but its the best for me now.  At expansion they are gonna start balancing  +heals to level of heals i guess below level 5 is what i have read.  So there is my two cents on heals and the likes.