WoW Insider recently wrote up an article that explains the potential gold benefits of having an account-wide pet collection.
I agree that this could be a way for players to earn some gold when starting out on a new server, and I think it seems like a smart way to establish yourself on a different server, where starting out might be tougher without some extra gold.
With your choices of what pets you can actually cage and sell/trade being limited though, your potential for game-breaking profits aren't too high, which means it's not like you'll be bringing down the server's economy with your influx of companions.
It would be like selling certain pets, as per usual, on your main toon's server. The market and demand would dictate the prices; the only difference is that the source of the pets wouldn't have originated from the server you're attempting to sell the companions on. And you'd be cutting out the middleman (Neutral AH), but that's not necessarily a bad thing, is it?
After thinking about it for a while, I really don't see a downside. Sure, already established players on certain realms might feel as though outsiders are randomly inserting themselves into "their" market, but that's going to happen even if pet collections weren't accound-wide. It might take a bit longer and be less convenient for that new neighbor in town, but it happens even now so it's not a new situation.
If there's any downside, it's that the player investing in the pets to possibly sell on other servers might take a gold loss themselves. Each server has its own economy, one that fluctuates and will vary from other servers. Buying low and selling high is always a risk, but if someone is willing to put their money on it and makes a profit, then more power to them.