The reasons for the lightening style is structure. As a whole, the entire module is only 8 inches tall, from the top of the power transmission shaft to the bottom of the tread.

The plates used are 1/4", and it can't be any thinner, as it's held together with bolts. As for sideload issues, Inventor 11's stress analysis tells me that it can take about 900 pounds of sideload as a whole, which just might be enough for a FIRST bot.