If we're talking about healing only, never needed MP. Lightspeed to heal only is lame, but at least we have something that helps with MP.

When I try to dps, I need a lot of MP top spam Gravity... But that's when all the tips above come to play.

AST has pros that other healers don't have: We can extend
-Largesse, so our heals will be stronger and we save MP in the proccess...
-Lucid Dreaming, so we'll regen more MP... (If we get to extend LD+Ewer, we're golden, let the DPS die, 'cause you can Gravity every mob by yourself...)
-I didn't test yet, but we probably can extend Eye for an Eye, can't we? More mitigation also means less healing...
-And Synastry - even if it lost the boost to healing - is still a plus. If you put it on the tank and heal him, it will work like a 20% heal buff, I use it more often than Largesse (It's almost like we had a fairie...)

So focus on optimize your Celestial Oposition when you're dpsing, you sure will love it...