Last post on the B-pillar from someone who has examined the problem in the past:
It isn't "necessarily" for stiffness but, in general, B's are key locators for torsional stress. This may not be the case here, of course. But given what I've seen and in my best guess, it is to keep the cost of this new model down. Before you blame "bean counters" (whoever they are) just know that pillarless coupes creak and groan and cause warranty headaches like you wouldn't believe. The only way around those problems is to add cost & complexity that add weight and raise the price. That means lower performance, lower fuel mileage and lower sales, all difficult to swallow in today's business climate.
With this car anticipated to be significantly more expensive than a Mustang (it's loosely based on an E-class Benz platform after all) and Chrysler's very life on the line, they have to choose their battles. I think this was a good and reasonable choice given the incredibly mild asthetic sacrifice and the hard realities of developing the car. Given those pressures, we should be thankful such a unique and inspiring vehicle is even making it to market.
So personally, the resurrected Trans-Am track battles of Challenger v. Mustang v. Camaro would be the battles I'd like to see them wage and this is a way of making that happen.