Unfortunately, NC is one of those things that is intuitively appealing to a lot of people--"I'm happy to give my stuff away for free but I don't want someone else to make money off of it". But it breaks down with essentially any non-trivial use.

Yes, using a photo off flickr as a screensaver on my home computer is clearly non-commercial but, really, who cares if that usage is properly licensed or not. Pretty much any substantive use could be argued to be commercial in some way, shape, or form.