It's all to do with marketing, as the old saying goes "horsepower sells cars...", especially in the market the SLK plays in where appearances and numbers mean more to most buyers than capabilities - not the case with the enlightened members on here of course!
But if MB doesn't keep the car "fresh" it's sales will soon lag in comparison to newer shinier offerings from competitors.
I agree with you wholeheartedly on the weight issue - the SLK55 could certainly shed a hundred pounds or two. And yes AMG seems to be little more than an afterthought with it's image being diluted through the usual body kit and lowering spring "packages", but I honestly think they're trying to change that too what with their new from the ground up engine program. That's more of a return to their roots from before they were bought up by MB, and I hope they continue to distance themselves from the body kit clique.