If used in the right context, I don't see a problem with 'zero rating.' T-mobile[0] has been doing this for a little bit now and, to me, it seems to be beneficial. The issues only arise when you are zero rating individual services belonging to one genre or vertical. Saying that Netflix won't count against your usage but Hulu does would be a problem.
[0] http://www.t-mobile.com/offer/free-music-streaming.html