Under no circumstances can the BMW system be better than Wave/Google Maps. One company manufacturers cars, the other makes routing algorithms and has an order of magnitude more sensor data.
Nobody has claimed it's better - it's just different. I've explained from my own experience the pros and cons I've found from each.
You don't think BMW themselves produce their own traffic data though, do you? I suppose you must do as you've pointed out BMW traffic will be worse because they make cars. They purchase it from elsewhere, currently:
The traffic information data of the BMW ConnectedDrive Service Real Time Traffic Information (RTTI) comes from anonymised movement profiles, GPS data of commercial vehicle fleets or traffic infrastructure. Currently, the service receives this data from TomTom.
Like I said, I think the main difference is the fact they do things differently. Google is a predictive service, BMW RTTI is not. There are benefits and disadvantages to each of these approaches.
If I want to know the best route through city traffic, Google is far better than RTTI. If I want sensible traffic information on a long distance journey, I find RTTI more useful than Google for the reasons I've outlined above.
As with most things, a combination of tools is better than just one. I tend to check Google Maps before setting off and when having a break to get a good idea of overall expectations and then use RTTI when en-route to respond to things like road closures or get information about upcoming queues etc.