There's a big difference between "information availability" and discussion. Blogs can point to a lot of references that may or may not be good information. They provide perspective and personal insight. They do not provide a good venue for dialog.
A blog is usually author-centric and hierarchical, where online forums tend to be networked and participant-centric (if that's not a contradiction in terms). The advantage of a blog (and what distinguishes it from traditional journalism) is that its author is usually an involved and passionate subject matter enthusiast, not an objective but superficial analyst. So you can find blogs that you buy into emotionally. In blogs you read unashamed opinion, not necessarily "information".
I think of blogs as personal broadcasting: the author(s) publish their thoughts and interests, and exhibit their expertise, to anyone who is interested. Those who are moved to do so can post a response. Sometimes this can lead to an extended conversation, but not often. Blogs CAN become discussion groups, particularly for small groups, though I have yet to see blog software that facilitates threaded discussion for a large number of participants.
The Internet has provided a vast distributed database of "stuff" -- some of it information, some of it opinion, some of it important, some of it trivial, some of it true, some of it false. Search engines are still primitive in their abilities to find the things you are looking for. Blogging is simply another way to find the stuff that you value, like having someone who appears to know what they are doing recommend resources within a context that you can relate to.
Discussion groups provide a different kind of experience. Much of their value lies in the pool of tacit knowledge and experience that is available to be tapped, and the ability to discourse about things that are not easily summarized in a neat published article. Most importantly, discussion groups are not hierarchical broadcast vehicles, they are networked -- which means that their potential power as a knowledge resource is exponentially greater than that of a blog.
I confess I spend much more time scanning blogs and RSS feeds than I do in discussion forums, which was not the case two years ago. But discussion forums like trdev or ASTD provide the diversity of opinion and a discussion dynamic that is still missing on blogs.