I won't lie, I have a few items in my library that aren't completely kosher. However, part of the terms of service for match are that you legally own any material that use the match service on. But nobody, including Apple or the RIAA is expecting that people won't have a few songs that they don't really own. But that is a different question than someone who has thousands of songs that came from torrents. It makes me uncomfortable when people begin to openly discuss flagrant copyright violations.
I doubt that anyone here is perfect, but that doesn't mean we feel ok discussing in an open forum how to do things that people have ended up in court over.
And I hope people don't think that they can use match to cover their tracks. Prosecution happens when they can link your IP address to an illegal file transfer. Possession of the file (or non-possession) usually is not enough to make a difference in court. So, if you think you can go on a download spree and hide your tracks with match, you may find the RIAA knocking on your door.