In an of itself, Napster and the gang pose a minor threat to copyright, because of the current volume of the thieving... I mean "sharing". However, if permission-less, license-free download of copywritten material becomes the norm, there will be a slow and eventual death to copyright.
My stance against Napster (because they did not pay the songwriter/publishers and artist/labels) is for this very same reason. One day it's just a download, the next day music is all free and musicians get paid by selling advertising for another company's "PRoduct". The present rationale that supports the Napster philosophy is "if it's invisible (digital) then it's free game.
It's the precendence that it sets. To me, that precedent is worse that the possible benefit that it could provide (as it exists today). Now if we change the model and Napsters actually pays for a broadcast license and pays royalties to the labels, while still making the service free, then I think you have something there.