Jim,
I don't know the whole history of Usenet and signatures and commercial ventures. But, what you are saying about including links makes sense. Adding a link is adding a link. Although, to me, there is a difference between adding a link after a message that adds to the groups discussion, and just posting a link to drum up business. Personally the adds that are blatant don't bother me. Sometimes I check them out. Most of the time I see that they don't have anything of interest to me. I do dislike the deceitful post that try to appear to be from an impartial customer, but come from someone associated with the website. And, I would hate to see this newsgroup get bogged down with advertisements.
I see the other point also. You mentioned that signatures are used to give weight to the poster's arguments. I think what is being proposed does the same thing, It may not add weight to their argument, but it helps us determine how much weight to give what they are saying based on biases they may have because of their commercial venture.
Blues