I'll make a point on it, although I'm not saying this why the size limits are as they are it's just something for you to think about.
If we let everybody have a banner-ad in their sig wouldn't that sort of undermine the paid-advertising that we offer here? I'd say so, fair enough it's not going to appear on every page but we are an active community so the chances are it would be seen just as much.
Anyway the rule is there, deal with it. It's not a hard rule to follow and I know there are some people that have started to brake it again but don't worry they'll get seen to soon enough