Firewalls don't influence what kind of results you'll get (maybe the amount).
It's probably because users that don't know much about P2P or comps (basically, people who are rather uninformed) help spread bad files (files with wrong filenames, fake files, etc) like the plague because they don't remove them from their HDs

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
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The Golden Age Of Leather"
-Blue Öyster Cult-