But even if you didn't, your firewall cannot protect you against the threats specific to p2p. The ipfilter contains a list of IP addresses of the known bad guys - bad servers, anti p2p activists, and government agencies etc. eg. If you see a client that has an IP address that is in a block of IP addresses that has been allotted to Sony Corporation or US Dept of Treasury, you know that client can be up to no good. The ipfilter automates this process for you.
The ipfilter I am currently using has 122908 IP filters loaded. You can do the same thing in your firewall by creating an individual rule to block each of the addresses in the ipfilter.