As they don't need as much downforce, let alone as much adjustable, as in F1, it makes sense to work only with the undertray you need only one design, integrated into the main design, even if as far as I know it isn't cheaper to do. It surely gives a cleaner air flux behind the car, so reducing the turbulence for the following cars and allowing for more competition. Sort of what F1 tried to do this year, even if they claimed it didn't work because of the "double decker" design, while it was possible to see both Brawn running hot laps quite close each others in the first races. What killed the competition are the usual pointless rules, horrible tires, and a lot of other reasons.
F1 could even consider it, even if that would significantly drop their performance as only wings can provide such a high downforce with the current F1 cars' design.
What they would never adopt is the unstressed engine, which I wouldn't consider for IRL cars either. Leave that for GT and Touring cars.