Actually, while you are in principle correct, in practice the risks are rather small.
For example, those searching a P2P net for music files have many sources from which to choose, and are able to see the characteristics of the files currently available & their hosts, such as Title, Artist, File Size, Bit Rate, Play Time, type of Connection (T-1, DSL, Cable, Modem), etc..
While it would be theoretically possible to design a file that could both masquerade as a legitimate music file and execute itself when "played," in order to propagate it would:
a) need to provide the expected musical content, so as to [b]not[/b] be at risk of being immediately purged by the recipient of such; and,
b) [b]not[/b] impair the ability of the now infected machine to remain sufficiently functional, so as to afford it the opportunity to be offered by its new host for sharing.
As this is no small feat, the use of P2P as an effective & efficient means of propagation is more suitable to files which are [i]expected[i] to be executables.
Therefore, one can quite easily mitigate the risk by being selective with regards to the file types selected; and, if deemed necessary, 1st quarantining them for closer scrutiny.
You can relax now; I was kidding about the "gotcha."