I'm not entirely sure signatures entirely need disabled. While I agree that sig images are pointless and signatures in general can be unwieldy, I also feel that length-limited (250 chars, perhaps?) text/link only signatures CAN be beneficial. Off the top of my head, people could link to their own mods/tools. This would be particularly handy for people whose projects are not frequently updated, as some really good stuff can get pushed back a few pages never to be seen again. If wasted screen space is really a concern, a smaller default size could also be used, and the size tag disabled.
Note: to be clear, I am absolutely not advocating or requesting signatures with images or any other stupid self-indulgent nonsense like that.
An example of a tasteful yet useful sort of signature:
A useful tool | Game mod | Another thing