Personally, I have yet to get a single virus from P2P so I am going to say that most people are rather paranoid, or at least uninformed.
That said, it is easy to get a bad file that is infected with something. The main reason I have yet to be infected is not that I run a better-than-average anti-virus program (many commercial AV programs are <95% effective, and some of the popular ones are <90%), but that I can separate the wheat from the chaff. The file sizes are a large clue, and there are other hints, but weeding out the bad apples really is almost an art form and thus not easily taught.
The truth is that P2P is about as dangerous as driving a car. With a little skill it is quite safe, but for the untrained and/or klutzy people it can be quite dangerous. P2P is a solid example of Caveat emptor, but given that many people seem perplexed by technology (and especially computers) many otherwise intelligent people get into big trouble with P2P and wind up downloading the wrong thing and not having a good enough anti-virus program to stop what happens next.
P2P is not inherently dangerous. It won’t give you a virus, but it will let you go out and get one for yourself if you tell it to. The trick with all things tech-related (including P2P) is to remember is that it will do what you tell it to, so make sure that what you tell it to do is what you want it to do. For instance, if you want to stop a car, most people hit the brakes. That will tell the car to stop the wheels from rotating, but whether or not that actually stops the car is another matter entirely, as anyone who has ever driven in the snow can tell you.