Large "block sigs", sigs which appear to be one large block of an image (Trotter, b1gapl, and Peter) take up lots of space on the screen, your eye is drawn away from the post and to the bright and pretty colours below.
Sigs with just plain text, or text and an image (such as backup, chris's and mine) are less invasive on the eyes (and take shorter load times)
However i doubt that most users (who spend alot of time one their sigs) would not want such restrictions imposed on them
I think we have to find a happy medium here, if we did go for the only one sig option, users on a slow connection (the ones we are trying to help) would see the large sigs and hit back becuase they couldnt be bothered to wait. Clearly this would be non members, dial up, coming in from a search engiene and not realise that it would only load once.
Tech-Forums members would also be a little more, how can I put this, "rebellious" and think that becuase their sig was only going to get 1 post in the spot light they would need to cram as much in as possiable!
Im not sure, definatly my text only mode could work, and the SE's would love it!