I'm not sure how I'd do acceptance/E2E testing without the magic of port 0. As long as you have a way of capturing what port it ended up using (which most languages are able to out of the box more or less), parallelizing testing workloads becomes such a breeze without having to manage ports and all that jazz, just fire everything up with N parallism and let the OS figure out what the free ports are, pass the references down and stuff just works. Thanks Linux :)