Sigs are an issue of user friendliness, IMHO.
We made many changes to the stock forum in the beginning with one main reason in mind. We felt it was not right to have our users scroll through long screens where the sig and avatar took up more room than needed.
Sigs were always limited in the old forum - and, we tried to limit them in the new forum but the software didn't allow it at first. Now it does. A BIG new issue is the percentage of users now on mobile. Just imagine the length of a sig on the small screens!
The new limits are for 3 lines. Look at mine. Not too bad, IMHO. Note that you can link in your sig to your profile page which can have a LOT more info...
The screen shot below shows one small example. Notice that the top post, without the long sig, would have fit in the same approx. space that the avatar block did. But with the long sig, the reader has to scroll about 50% more....
Are such details something which I expect the average poster here to understand and agree with? Actually, no - but I think if you ponder it you will see that we aren't doing it for punishment or to limit expression, but rather in one more attempt to provide the best overall experience to the readers.
Also, notice the statistics on the side - most readers are NOT members or posters. Not to say we don't love members more, but the users get a vote too. They want info as condensed as possible.
Yeah, I know I am a little obsessed about these thing. So was Steve Jobs, I'm sure!