Oh

"Can't see any benefit in blocking outgoing ports whatsoever myself"

You don't see any need to prevent internal network data from leaking out onto the internet? An interesting point of view.

Leaving aside the very obvious security considerations for now, it is this thinking that has resulted in the small but significant amount of noise on the internet at large, consisting of stuff like Windows broadcast requests searching for other machines (which it will never find), and other stray broadcast message. All of these messages should never leak out; they should be contained on the local network. And before you say "it doesn't matter", well, yes it does! It wastes bandwidth and causes load on other internet-based kit that has to deal with this stuff.

And contrary to what you say, many expoits rely on ports other than 80 being open.