Well, my dial-up software does this (yea, I'm cheap and still use it!) and it makes complete sense. Since the port is open and in use, what good would launching another instance of the dialer do? Nada. Saves an old fool like me from receiving an annoying alert-box telling me that my modem is already in use (which I had received a few times anyways when the window was minimized and I simply forget I was already signed on).
This may be a lousy example, but as you say, there probably isn't many valid examples out there.