I think the danger lies in referring to a band or even a particular album as existing within only one genre of music, and that's where people oftentimes 'make' a genre more nebulous than it actually is. (Though let's not forget that genres are already pretty nebulous in the first place.) ferday touched on this with Swans above--we may now think of Swans as a firm post-rock band, but they came out of the same NYC no-wave scene that Glenn Branca and Sonic Youth did, and you can trace that heavy noise-rock influence throughout most of their work. Their newest album, The Seer, is as much a drone album as it is a post-rock album. Even GY!BE, the 'ultimate' post-rock band has heavy drone, ambient, and chamber leanings, which all can and obviously do exist independent of post-rock elsewhere. (And, to throw a spanner into the works, like a lot of post-rock bands, they heavily self-identify as 'punk rock.')
So, now that I've restated everything ferday said above, just in my own way lol, I've gotta ask--any 'post-hip hop' fans out there? I figure that if you take what the 'post' means in post-rock (or what it feels like it means, if we choose not to quantify it), that you could apply 'post' to any major genre and find examples of that genre that work 'within' the expectations of the 'post' label. For example, listen to the album Absence, by Dalek. Hip hop yes, but in my iTunes I also have it marked as 'Ambient' and 'Noise.' Just like it's been mentioned that in post-rock the guitars are used to facilitate texture rather than their 'standard' use, so the 'beats' (such as they are) on Absence are almost all texture and very little traditional hip hop. Just throwing that out there. ^^
