To accomplish a downward force on the track between the rear/front drive/idler does not require idler wheels. A rail (similar to older snowmobile designs prior to popular use of bogey wheels) will accomplish the same thing. It has to be designed right so as to not cause unnecessarily high amounts of friction. Without running numbers my hunch is that the bogey wheels are more efficient in the area of reducing friction.
With regards to the logic of using treads versus wheels - I think this year's game is a perfect example of the usefullness of tracks. Why? If your team wants to climb the ramp to score the points at the end of the match you need either ground clearance to the tune of roughly 4.5 inches on a 38" long drive with 6 inch wheels. This approach causes two problems that our team has identified - less space in the 28x38x60 box for robot "stuff" and a higher center of gravity. Tracks eliminate the need for ground clearance by providing contact all along the robot.
With that being said, our team is still using wheels this year but in a slightly different configuration than years past. Our team has not put much effort into tracks and if I keep mentioning them, Mike Johnson will kick me again!
My 2 cents - take them for what they're worth.
Sean