I think its redundant to have two conversations going on about the same thing. Comments on the blog are comments on the blog - they are strictly moderated, and they are almost all always directed back to the original story, etc. Whereas on the forum, its more of a discussion, with the topic often drifting, the OP participating, providing feedback & more info, etc, and comments moderated very differently.
I think its almost always impossible too, to keep from having the blog comments influence the conversation if we move a topic over to the forum.
And with the info that banned-from-the-forum folks are commenting on the blogs, it seems a bit unfair to essentially take their topics as our own. Banned folks can still read what we are writing on the forums even if they can't participate, so it comes across as very "ha ha we could participate with you all but we'd rather have our own private conversation you can know about but that you can't participate in! Oh but we might still butt into your conversation too, cause yours is public but ours is exclusive!"
When we have duplicate threads on the forums one is generally shut down or left to die - like if someone reads an outrageous Dear Abby and posts it in Life in General and then someone else posts it in Entertaining, someone will say "there's already a thread on this" - well the same with blog posts being reposted - there's already a place to comment on this: the blog.