My friend got rejected from her top choice program, and she emailed her POI just wondering why so she could take that into account when reapplying. About a week later, she got a really detailed and helpful response as to why she wasn't selected. It was a bummer for her to have all of the things "wrong" pointed out, but now she knows what she needs to work on. I think it really varies on the school and the program- you might hear nothing back, you might get a generic response, or something actually useful.
If you're going to ask, do it tactfully. Don't sound accusing, angry, disappointed, etc. Good luck!