Yeah, I don't really see the reason for the at all in a forum. They seem to have been carried over from email and USENET for some reason. I've only rarely used one here, and if I use one on other forums usually it's just a simple, one line signature that links back to AtariAge. Some forums are really out of control as far as signatures go, and you're not kidding in terms of that 90% signature and 10% message content ratio on.
..Al
One useful purpose for a signature is that it points toward something current -- even on old posts. It can be helpful in some forums, for example, to be a pointer back to a FAQ that the poster maintains.