Recently, someone that I mentor informally sent me an IM. She said, "something curious happened today. A manager from a another unit came over and asked me to apply for a promotion in his area. What shall I do?"
On most subjects I would replied by IM, but this time I picked up the phone and almost shouted "do the interview!" She said, "but I already have a promotion lined up and also, I expect my boss to leave soon and I should get his job".
Let's go through the reasons I was so adamant that she should apply:
Some years ago, I had a vacancy for a DBA manager in my department. One of the senior DBAs applied. I did the interviews with my boss. When he heard that we were interviewing this guy, he said, "why are we wasting our time, he isn't getting it". There wasn't time to discuss it so I said, "let's do the interview just as we have done the others and see how he does".
After the interview, my boss said, "he surprised me, he has a lot more to offer than I thought". Then he added, "shame he has upset some key people, we have to think about how we repair his career".
He already knew that he had little chance and he didn't get the job, but he did get a VP on his side. So the interview was worth the effort.