What isn't particularly distracting for you is a painfully detrimental experience for any of the hundreds ADHD/OCD/Schizophrenics that roam the forums with fingers sticky from spilled energy drinks and pixie sticks.
Essentially, it's just easier to ban animated signatures than to examine each and every one to assess whether it'll distract in any way. But, if you want a continuously switching signature, just write a php script for one.
Better yet, I have an old one lying around:
<?php
$signatures[0] = "ieatcheese.jpg";
$signatures[1] = "andhamtoo.gif";
$signatures[2] = "ittastesgood.gif";
$signatures[3] = "nowineedtopoo.gif";
$url = 'http://llsc.us/members/yourname/uploads/'; //The directory of your pictures
$signature_number = rand(0,3); //Replace 3 with the number of images you have
$rand_signature = "$signatures[$signature_number]";
header("Location: $url" . "$rand_signature");
?>
Someone around here could probably write a more user-friendly script, but it's not entirely difficult to set this thing up. Just put all of your images in one directory, fix the names of the images with those on the list (you can add more lines if you need), fix the url for your directory, then make your signature: "[img]http://llsc.us/members/yourname/uploads/phpscript.php[/img]."
