Except for the plastic keyboard, I don't see any advantage. Use a decent Jabber client (I'm using Beem on Android) and you get all these features on gchat, with just your gmail address.
Plus, figure out the priority thing right, and you can have all the messages delivered to the right client. GTalk broadcasts to all clients by default, but the better IM clients give you an option to send and receive.
Couple of reasons people like BBM so much (observation of BB friends):
1) You can see when someone is typing
2) It's very fast and feels like your talking with someone on gchat/aim/etc
3) Most BBM users are already "locked in" with eachother via BB PIN #'s. It would be like changing your phone number if you got off BB
4) A person in the US can BBM a person in the UK for free