Its not really the size, or indeed the problems to edit if something goes wrong - it probably never would - the problem is that that forum is not a Computer Forum; my Forum allows that, Matt's Forum allows that, we've all seen Forums with over 6,000+ Members with Image Signitures and Custom Avatars - but they're not ComputerForums. David has a sort of thing here which is different in that its a clean cut environment; its not really an entertainment Forum, its for people who know nothing about Computers to ask for help.
Image Signitures and Custom Avatars would change that feeling, and turn it into a place than the simple folk who come on here merely to ask for help would not feel like they could ask anything. It would quickly become very elite and look like we're not approachable.