Depends-- I won't always do it... but if I'm in other battles I want to win, or if I'm trying to avoid them, yes, those would be solid reasons.
certainly nothing to do with courtesy. Sometimes it's better for your opponent to think you're going all-in on 1 guy, sometimes you want him not to know who you're fighting for (or even how many). It's all about people perceiving your position as strong.
In the case you're citing above, where a 3rd team ends up with him, you've erred by not simply withdrawing the "inform of" once the guy is available again. Your previous effort, assuming it's significant, would still weigh in a lot, and you could still sign him. All that depends on the timing of everything, too, and when you've given up the battle, he's decided not to sign, what other efforts you've made, etc etc etc.