Well if you don't want to DPS it's okay, you're a healer after all but it may be required in really extreme DPS checks in endgame raids. To be honest, you'll see that even keeping everyone topped and shielded leaves you plenty of free time after you reach max level and have a good gear.. you'll find yourself wanting to DPS to not get bored LOL.

That said, SCH have fairies healing machine and nice DoTs and all so it's pretty intuitive that you can DPS, WHM and AST are less but they give you so many powerful offensive skill as a WHM and you can even keep 3 HoTs running at the same time healing people more than what they need so yeah.. what to do what to do? You can jump around or you can DPS!

AST is the way to go if you don't want to DPS, you can DPS with that too but other than your healing kit being a mix of the other two, leading to not be a master of neither, you have an internal party buff mechanic that fill your time, still you have a powerful DoT and a cool offensive AoE you may want to use in the right timing.