This move would be sensible. I can only see Harper as a liability to Ryan's system. The guy doesn't like sending dbs, he puts them out on an island. It doesn't make sense to keep a safety who cant play man-to-man.
Good coaches play to their players's strengths. I don't think Rob Ryan would ask him to cover all the time. Would it be silly to have him as a backup safety at his salary? Maybe....but 9 sacks from 2011 tell me that he can be effective, when used properly.