We have used the kit frame and 1' square tube or 1/2'-1.5' angle for nearly everything on our robots, from 05'-10'.
We have been very successful with this method.
Each year we have built the fram in two pieces(a drivebase and an upper structure for everything on top) then either welded, bolted, or welded and bolted(07') the two pieces together.
It's easy, cheap, reliable, quick, light, and very strong, plus we dont really need any machine other than a saw and drill to build everything*
*welding was done by a parent or mentor