Like Cyberblue, we used extrusion in the past (except for the "pre-me" robots - Rowdy 1 [2000] and Rowdy 2 [2001]). This summer, though, we replaced the frame with a welded box (1 in by 1 in or so) and it worked perfectly. It was 2 lbs lighter and we had weight to put on a pump (essential to the arm). Even though we didn't do great at IRI (because our arm broke in two places during the practice match - took 2 hours to fix, and the whole energy left in a battery issue), the frame was definitely a plus, and welded frames are what separate the men from the boys.
An example: Teams 66 and 279, the awesome teams we allied with at nats. Though 279 did use some extrusion in their arm, both 66 and 279 had welded frames (as far as I could see). They had very few limitations, except when the opposing robot would knock it over by entangling with the arm.
There are many more examples of this. Good one: WildStang this year, Beatty 71, and Las Guerrillas (469).