I'm not sure what a healing dummy would help with? It's relative to each person, for some people most of the difficulty in healing comes with learning how to weave heals while DPSing and evading mechanics, not sure how a dummy can help with that- with the exception perhaps of practicing slidecasting.

If it's to get used to any distance limitations, there's that too. other than than healer rotations are so simple, it's not like DPS or even tank rotations where DPS dummies really help building muscle memory.