The two plugs per cylinder idea has been around for years and years...since the 1920's, I believe. The design was more commonly found on the European cars (Alfa Romeo liked to do this a lot), but found its way into the American lineup as well.

The idea behind it is that with twice the spark you'll get a quicker, more complete burn. Whether or not it produces more power is questionable, but you do get an increased throttle response. Additionally it aides in increasing fuel economy and reducing emissions.

Is it really necessary? Probably not. But we're talking Mercedes here and why charge for 6 plugs when you can charge for 12 instead?