That's just it, a true "unibody" is stiff enough to stand alone without the use of a "B-pillar". But of course adding a b-pillar would add to it's structural integrity, and it's also a less costly method of strengthen the chassis.
As for the Cuda, if it ever reaches fruition, it would be saddled with the same chassis as the Challenger, so it won't be a "true" hardtop either.
I'm "old school", and I just think it looks cooler when all the windows can go down. It gives you a much more "open air" feeling.