You also kill the queue for whomever waited for it to pop so there's that time lost for them. Now they can choose to wait in a "dead" pop and hope backfill works and have nothing to do in the mean time (if they were just in queue they could do other activities) or leave and re-queue and possibly get another person like you. Everyone does not win.
You most definitely can choose when to mentor and when not to mentor. It is not a job. However when you choose to queue "mentor" queue, you'd think you'd be choosing to mentor. You're using a mentor resource with a mentor based purpose.