If we don't have a roof with retractable sides to the stadium, you don't play the NBA All Star game there, nor the Final Four, or various other things that will inevitably be signed.
Jerry didn't drop $1+ billion on the stadium just for the fun of it, it's an investment.
At least it's going to have the feel of an outdoor stadium. Hole in the roof, the sides open up so you'll get a breeze in there, and also all the glass around the building will help light the place up during day games.
The thing is amazing and I couldn't be any more proud to be a Dallas fan when I boast about the stadium. Anyone take a look at what the Giants' new stadium is going to look at? Seriously, we make that thing look like a piece of crap, and that's getting opened next season.