I think the point to remember here is choice. I personally probably wouldn't opt for a diesel cabriolet just yet, but realistically if it's what people want, what does it matter?
If someone wants to buy such a car, and Mercedes-Benz don't offer one, they'll most likely look elsewhere (BMW, Audi, etc).
Only a few years ago, domestic use for diesel cars seemed unjustified - and even more recently, diesel coupes were rare sights. Today there's plenty of BMW, Audi and now MB coupes out there (wasn't the 209 CLK the first step for MB here?)
Will