I think there can be all sorts of reasons for regression, even for play, that aren't necessarily negative or meant to hold a patient back. For example, I believe that folks who aren't all that in touch with who they used to be, or can't really even remember anything much about a traumatic childhood could probably benefit from anything that helps them feel those feelings again.
Do I think it would be good for me to sit down and play with a doll every week and for my T to be my "new mom" or something? No...I DO think that would be encouraging me not to grieve and grow. But can it be useful in a limited way, as in the above example? Could it even be the catalyst for grieving? I'd really think so.