IMHO, there shouldn't be any advertising (or referral links) to any other sites other than direct links to FlyerTalk in peoples signatures.
IMHO, (and emphasis on the IMHO) the reason the rule was changed is because it wasn't consistenly enforced and some mods just didn't seem to care if people were posting things in the sigs that other mods did (at least that's what it seemed like).
Personally, I think they're a bad thing.