SCH + AST is the top party buffing combination for DPS. A2 clear with AST
https://www.youtube.com/watch?v=FWfPcG43yrA for example. If your party composition is not optimal for Selene (*cough* SMN *cough*) and it runs up your TP bill, or you'd much rather time DPS buffs for burst phases rather than a passive haste, AST works better. It's the only healer that can grant TP/MP. Synastry is amazeballs. Single target AST DPS isn't too shabby. Perhaps the WHM isn't god tier and can't shoulder all of the healing burden. AST would be contributing DPS anyway, where my SCH would be babysitting.
How about, it's fun?
AST is not perfect right now, but a lot of the complaints about it that I've seen are a bit off base.