When you box a channel type frame, it increases the torsional rigidity of the beams themselves, and reduces the tendancy to bend at the ends, (or sag in the middle).. The cross bracing adds to the torsional rigidity of the frame as a whole, but adds little to the resistance to bend.
Also, all of the pickup points for the suspension and motor can all benefit from being attached to a boxed frame member.
Being as how there is alot going on in the front of the frame, as well as the back, why not box the whole thing?
That's what I think , anyway.
We really need some REAL structural engineers to answer these kinds of posts.
I know there are a few on the board.
my signature lines...not really directed at anyone in particular..
BE different....ACT normal.
No one is completely useless..They can always be used as a bad example