Of course give it a go. The card system is nowhere near as good, but overall AST is a solid healer that can easily cover both mitigation and healing and provide more overall DPS than even a WHM.

Also it's worth noting that MP issues is only a problem when DPSing heavily, which I personally suggest every healer do, but the fact is a lot don't, and if you just like to heal and throw out the occasional DPS spell then AST is a fine MP-wise.