I don’t buy the whole ‘German driver winning is best’ stuff. If Rosberg wins the WDC then they sell more cars in Germany. If Hamilton wins they sell more cars in the UK and, more importantly, the US. He has a much higher profile in the wealthy young market that buys expensive cars, hanging out with other stars and appearing at fashion events for example. This makes the cars appear ‘cool’ and fashionable, Rosberg probably appeals more to their traditional market of middle-aged golfers.
As long as Merc have a winning car with WDC driver in it then they’re happy. Having 2 WDC drivers may be better but it could also cause internal problems.