As well, the current screen uses a masking system. For good masking performance, the masks need to be as close to the screen surface as possible. With two screens, one screen will suffer.
The entire theater is automated. Shades, lighting, screen drop, screen masking. Aspect ratio adjustments are automatic depending upon cues from the movie server. So, to this fully automated theater, I need to add a step to get up and walk over to the projector and tweak the focus ring depending upon type of content? With infrequent 3D viewing, that really honestly probably fine for me since my projector location is easy to access. It'd be a problem in some installations though (perhaps it's built into a soffit, for example, as several folks I know have theirs).
I'm not saying that there are better options at this price point. There aren't. The only 4K alternative I'm aware of that's even remotely close is the Sony VPL-VW1000ES, which is $24,999 MSRP, can't do 4K at anything more than 24p, and is not as bright.
So, like I said, I really congratulate Red on what sounds like an amazing and groundbreaking product, but it's really unfortunate that it requires a special screen. This may seem like a non-issue to people who really don't do much home theater, but if you take a stroll over to some of the home theater forums, there's growing disappointment because this requirement rules the projector out for a lot of people.
As I suggested above, Red could remove this objection by offering an option to use active glasses. That opens up all other screen surfaces and allows people who have a big investment in their existing rooms to exchange the projector without an expensive and disruptive replacement of the screen.
[Edit: Plus what Jonathan said. ]