Originally Posted by kooper
I think it's a) because the doors will have frameless windows and b) to add torsional rigidity to the body.
From a driver's perspective, I doubt the thickness of the B pillars will have much effect on the line of sight.
MB does frameless windows on many of their cars without such a thick b-pillar. Having driven a few rental sedans with thick b-pillars, I find it to be a terrible blind spot, as it's right where I look when looking over my shoulder. Regardless, this whole class of car, CLS, A7, A5 Sportback is generally pretty ugly in my opinon, the 6 GT being the first car in the class that I like (at least in pictures, I haven't seen one in person yet).