I guess this is the same train of thought as a Camry is basically a Lexus ES330 or a Tahoe is basically an Escalade, or a 4Runner is basically a Lexus GX470 , a Pacifica is bsically a R-Class Benz etc etc etc.

Platforms are shared across brands and models all the time, some with very little changes (think Toyota LandCruiser and Lexus LX)

It the little difference in details, materials and craftsmanship that sets the models apart. In this case, the folding hardtop of the SLK for one that sets it apart from out Corssfires. Well, even the properly designed cup holders in the SLK sets it apart from the Crossfire's cupholder..