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Forum Moderators: rogerd
The main reason here at WebmasterWorld is that we have strict self-promotion policies, and sigs tend to be self promos. Not all forums deny members the ability to self-promote, of course.
Ultimately, it's a forum-by-forum decision.
break up the flow of reading threads
this is actually a much bigger factor than people tend to think - big sigs really dilute the content of a page. You want a reader to keep reading and stay hooked til the bottom - it's not just kids that have a short attention span! a thread reader will be more involved in a thread if there aren't loads of sigs breaking up the flow - and if he's more involved, he's more likely to post. And that reply will more likely be a longer, more thought-provoking response, and not a lazy oneliner
I personally dislike sigs as it encourages worthless, "Hi, I'm new here! Great forum!" posts, with the sig... and you never see them again.
That kind of post adds nothing to the forum.
Allowing these people to spam my forum might increase my post count, thread count, and membership numbers, I feel it doesn't make my forum a better community.