If you notice on the CLK or E cab all four windows can be raised, and those are larger windows (i.e. more weight - more stress).I have to ask, other than the proverbial "forbidden fruit", why would you want to raise all four Windows with your top off? Better wind resistance? Perhaps that is the very reason the designers have disallowed the practice. Perhaps the rear windows simply do not have the tensile strength (for lack of a better term) to ensure that under harsh conditions, they wouldn't break. It makes perfect sense to me.