Without going into whether code should count as lines (such as when changing colours and such) an automated sig limit is quite difficult when the limit is 5 lines or 2 lines and one pic. I have asked before: 5 lines on what screen size? My wife's laptop's screen is a 17" widescreen. You will get more words per line when viewing the site on that screen than you will when looking at an old VGA screen set to 600x400. Also, I never have firefox maximised when viewing the internet so that other things I have open (like chat) are still visible. Again, there is the potential for variation in sig size.
Before strictly enforcing a limit, we should probably deal with actually defining what the rules are less ambiguously.
Finally, there are mods who would lose their sigs if the sig rule was enforced strictly, or a limit brought in.
Yes you're right, but are any of us going to be that picky as to moan that someone else has slightly more space than the next person? If I ever felt so incensed as to start a thread complaining about being treated unfairly in the sig stakes, then I sincerely hope I'd of come to my senses before I got round to hitting the submit button...
Would a character limit rather than a line limit resolve that problem?
Assuming there is no solution that is totally fair to all users, then surely setting it to the most common standard is the best approach.
Finally, there are mods who would lose their sigs if the sig rule was enforced strictly, or a limit brought in.
I fail to see the problem.