I'm not sure what my team's final decision on chassis material this year will be, but last year we used a plywood chassis because... well, have you ever tried to make those nice straight pieces of kitbot frame be round?
The wood construction was lighter than an equivalent metal frame would have been, and didn't impair the bot's driving and performance.
We use the kit frame whenever we want a more conventionally shaped robot, though.
Here's a pic from last build season of a prototype bot with basically the same construction its better-decorated descendant who played at the Oregon regional.
http://www.team847.com/uploads/image...d/DSCN0052.JPG