Yes it is viable, but the question is which one is more viable, all 3 healers can do the job. Which one is the harder worker? the one you play best and have researched/learned how to play optimally.

Schs need tons of macros, macroing the fairys heal into your own heals, macroing this, macroing that, but once you do all that sch turns into a healing mad house. I think I have 7-8 macros.

My whm is 45 or something but from what I have observed in parties (and raid), is if the whm feels that the sch prefer to heal only, then the whm is happy to spam stone 3 and off heal with medica 2 spam.

Sch is in a big place, whm is supporting the sch, but whm can main heal if sch want's to do dmg all the time (im a sch who only likes to heal).