Other MMO's have done it, but that's not why I think this would be a nice feature. It would be cool if you got pets for doing certain things, such as getting a war hound after helping a dude out.
They wouldn't serve a combat purpose, they'd just follow you and... be there. Sounds pointless, but it's just another little thing some people would like to use. Seeing as they wouldn't serve a direct purpose, perhaps there could be a toggle which makes them not appear on your screen if you don't want to see a horde of cats at your local guild.
They don't have to be normal cats, either. How about a baby troll or even a small cliffracer hovering overhead? You know, something like that?
This is just a thought and I highly doubt this would be in the game, just curious as to what you guys would think if pets were a part of the game.