Forum Moderators: open
Any useful info you might put in a sig file can be posted in your user profile, so it's not that the information is "forbiddden" from the boards entirely, though.
(I've also noticed that forums which allow sig files just tend to look cluttered, and the extra, off-topic text in all the posts will increase page size and loading times, which increase server load, and decrease overall board performance time...)
<I've also noticed that forums which allow sig files just tend to look cluttered, and the extra, off-topic text in all the posts will increase page size...>
Quite right. I've also noticed boards that allow HTML links suffer high signal to noise ratio. They appear to contain a high number of rubbish posts, probably due to misguided link building attempts.
Many other boards allow, even encourage them and that is their choice. Personally I like to control my links, a board gone to the bad is not a place to have a lot of links to your site, stay in control.