Without directly joining the fray, I'll toss in a couple of thoughts on the matter. Not all of Europe is grey, cold, and raining. Ever been to the South of Italy? It'll give the Southern U.S. a run for heat and humidity any summer. And of course, we all know they sell lots of big black Benzs down there.
As well, "driving" in Europe is more likely to mean stop & crawl city traffic than in the U.S. Population density in European cities is extremely high, and in many instances growth controls mean there are no suburbs. A good friend of mine performed a population density study on Munich for the purposes of planning wireless phone coverage. His finding was that density was constant (and very high) from the city center all the way to the edge, where it abruptly dropped to almost zero. Traffic followed essentially the same pattern.
One last comment, and this is perhaps the most significant. Japanese cars do not run with the engines at constant temperature. It is essentially impossible to build an automobile where the engine maintains a perfectly consistent temperature. Both the rate at which heat is removed (due to air flow across the radiator, temperature and humidity of that air, and coolant flow rate through the radiator) is highly variable, as well as the rate at which heat is produced, roughly corresponding to the work load of the engine.
What the Japanese have produced instead is an "idiot guage." The temperature guage in every Japanese branded car I can recall points at the same spot on the scale for all coolant temperatures between ~100F up to ~230F. Get yourself an infrared thermometer and see for yourself. (They're great toys, everyone should have one!) The guage points only to cold when the engine is stone cold and unable to provide any warmth via the heater. The guage points above normal only when the engine is about to overheat. Anything in between is considered normal and resolves to a single point on the guage.
So I see the matter not so much that Mercedes cannot design a good coooling system, but rather that many people have been "conditioned" by being fed false information. Mercedes is simply giving you a bit more information. The same issue is true of oil pressure guages - Mercedes gave up and eliminated it; Ford went to an idiot guage years ago.