If you receive a piece of music and listen to it (or remix it, or cover it, or…) you are a User of that music, and are free to distribute it. If you do not listen to a piece of music you are not a User, and do not strictly speaking have the right to Distribute it.
A recent survey showed that music users regarded sharing music that they have purchased with their friends as acceptable but that they regarded large-scale commercial copying without paying the original musician unacceptable. That is they supported Distribution by Users but not Distribution in the absence of Use. This is the difference between Use and Distribution as a social norm.
NC may seem to reflect this norm, even in its tolerance of peer-to-peer filesharing (which is Commercial Use under US law). Peer-to-peer filesharing between individual users of music allows them to share music that you have bought with their “200,000 closest friends” as Lessig puts it.
But the mechanics of a peer-to-peer service, or a repository site, amount to Distribution without Use. The GPL 3 draft has been modified to reflect this, allowing distribution of GPL licensed binaries through peer-to-peer systems. The personal use of a peer-to-peer network or a repository does not break the norm of Distribution by Users. But the monetization of such systems, the only hope Web 2.0 has of turning a profit, does.