As far as I am aware there are several reasons why file sharing is banned.
1, It causes LAG. Example. You leave file sharing on. You are playing a Game. Somone comes in and grabs that 1gig d/l off you. It causes your machine to LAG out

. Not good.
2, It prevents the sharing of software that is not ment to be shared. IE no warez.
3, There has been several occurances of people turning up to the LAN that do not have a virus checker. You would be surprised how quickly a virus can be passed round. hmmm Monkeyboy and having to do a low level format rings a bell.
4, I have found many a time people leaving their whole drive sharred on thier pc. This then can be very dangerous cant it

. I mean who would go in and put silly named folders on your desktop.
There is a Patch server with all the latest patches/mods/demos/maps for almost any game you can mention. All have been checked for viruses and at a fast d/l.
I hope that answers your questions. No to file sharing basically.