Maybe I am
I'm not sure I disagree with you. The super-nerds (as I'll call them) did well in school, and I'm sure make for great scientists. There are lots of hedge funds that would love a super-nerd to be their quant guy. Look at Renaissance, they're all PhDs in Physics and Math. But, then again, maybe the PhD is the better degree for these people.
Part of getting an MBA, at least for me, is to round out these social skills. Masters degrees in general are usually terminal, that is to say that the student goes and gets a job afterward. Schools like their job placement stats to be very good, and super-nerds can be hard to place. They're out of place in many environments. You're at an IB, right? I'm doubt your best quant is the same person closing the multi-billion dollar deal, even if his work was crucial to getting the deal done. In my mind, the MBA gets you the skills you need to be that closer, and the PhD does a lot of the heavy lifting along the way.
I am by no means saying that the super-nerd can't ultimately be successful, because I do believe they can. (I have a few friends who I'd consider to be in this group and I think there's a good shot of any of them having a spectacular scientific impact) I just don't know that B-school for an MBA is the right venue for them. I also think the underlying reason they're weeded out has to do with job placement.