I would make one practical argument and one impractical one:

1) They weigh a fuckton. Which means that yeah - you can make them go just as fast and handle just as well but you have to try twice as hard. A p/w ratio in a 6000lb car comparable to a 3000lb car requires twice as much power. That means twice as much engine. Horsing it around a corner requires twice as much tire. These are solved problems, obviously - there's many a Bentley with no difficulty and an f'n Maserati Granturismo tips the scales at an astonishing 4300lbs - but a CLS55 at 4300 lbs is simply 1.5x the car of a Porsche convertible at 3000. The AMG has the advantage of spiriting you and your three buddies around in total comfort but really - fuck my three buddies. I need room for a companion and a toddler.

2) While neither of my two friends are members of the Armenian mafia, their cars most assuredly are. Anyone who has spent any time in Los Angeles has formed an indelible association between AMG and Brand Blvd. in Glendale. Which is not AMG's fault, which is wholly irrational, but there it is.