To me, alternative just seems like a lazy iTunes-esque way of classifying music (Over half the CDs i put in are often listed under genre as "alternative"). Literally every genre of music has an actual name for it, or at least an alternate name besides "alternative".
Edit: Now that i think about it, there are a lot of bands that I like which are really unclassifiable. Sonic Youth, Pixies, Metric, Husker Du and others have so many different genres of music all fused together, so yeah, I think that "Alternative" is a viable genre of music, as the poster said below me.