The issue I see with calling them
sub-genres of dubstep is that while they might have stuff in common with dubstep, dubstep itself is too distinguished and specific of a genre to ever encompass tons of mini-genres. Rather than differentiating
this and
this from something like
this, it all falls under the flag "dubstep" - even though they sound nothing alike. The term "dubstep" is much like other broad genres pretty much meaningless outside of categorizing music. The word does tell you about the structure of the music, but it says nothing about the sound, the tempo or anything else. There's no rules set to control
how it sounds, and it shows more in electronic music than it does in regular geeh-tar music (pretty much entirely because of the usage of DAWs over the by comparison incredibly limited regular instruments.
Luckily however, there are very few albums that are limited to only a single genre. The James Blake song I linked for example, could be labeled with "dubstep" just as well as just "electronic", "post-dubstep" and "uk garage". I see the term "chillstep" being tossed around a whole lot, and I really don't mind. I won't ever call it a proper genre, but it does help filter out and isolate a very similar-sounding type of music. Which is what genres pretty much are, anyway.