the point with all the mobility talk is: a lot of people cant play this spec very well. the average pyro merc on my server does nothing else than IM - TD - and then just rapid shots 24/7. sure, they can run around like that (mostly they are snared though so dont ask me why they still try to what they call "kite"), but their dmg while doing just that is totally abysmal. they are basically a running joke on 2 legs.
to play the spec effectivly you HAVE to stand still and cast; not as often as arsenal, but you still have to do it. but again, the amount of **** pyro mercs running around spamming rapid shots is legion.