I actually think that most comments add more noise than information, and therefore decrease the value of a thread as a whole. This get worse as commenting gets easier, because easier posting means more comments, but most people don't actually have something interesting to say/write. Information gets diluted and less accessible/visible.
In fact, this applies to the whole Internet and I sometimes wonder how much time it would have taken to learn the same things I learned while browsing through tech-related blogs by, for example, reading books (which are, often, denser in terms of information).