They're pretty rare. I always heard from people that there were no unmatched PM&R programs the last few years, but I recall seeing the list of unmatched programs by specialty and there were a few (very few) PM&R positions that went unmatched.
Sometimes spots pop up, even at quality programs. If I recall, Mayo had an opening a while back. So did VCU and MCW. I think far fewer people leave PM&R than other fields, but it's still going to happen every now and then.
I don't believe programs hold spots for people who switch. I don't think it would be the wisest course of action for a program, unless they genuinely don't want to rank anyone who applied to their unmatched position through SOAP. But I could be wrong.