NC doesn’t protect artists from losing value from non-sales distribution.
NC explicitly allows both internet-era filesharing and walkman-era personal copying. NC therefore gives away any value that a direct audience for music can actually want to take. And it is as powerless as standard copyright to stop organized illegal commercial copying.
NC doesn’t protect artists from being ripped off by middlemen.
Artists who use NC can still sign to a record label. Which NC then cannot prevent ripping them off, failing to promote them, or dropping them. Magnatune clearly state that they are not evil and make clear and public the financial terms of their relationship with their signed artists. This is far more important than NC for artists not being ripped off.
NC is not the dissuader in NC-SA, SA is.
Magnatune will sell proprietary licences for music by their artists to film producers who wish to use the music commercially. But it is unlikely that those producers would SA their films if the music they wish to use was SA-only. NC on top of SA is simply redundant in this case.
NC is not Magnatune’s secret sauce.
NC doesn’t stop artists losing sales, being ripped off, or losing proprietary use revenue. The secret sauce in Magnatune’s business model is their strong and public ethical position, and this is far more difficult to reproduce technologically than their use of NC.