Richard,

Here's how I looked at it. In order to tie that long diagonal to the rear of the chassis, Pro put a bend in it. I felt that a strong upward force on the rear of the chassis would try to increase that bend, since the force was not acting in a straight line up the axis of the tube. The lower vertical brace keeps that from happening. Think about what a plain ol' steel gusset near the end of the tube would do. It effectively takes the bend out of play.

You suggested something that would accomplish about the same thing when you recommended making the tube as straight as possible.

Make sense?

Concerning the contribution to structural integrity, sometimes braces or stiffeners do as much for the structure as the compression and tension members. Think gussets again.

Bear in mind that I'm not a structural engineer (although I did a lot of similar stuff in the pipeline industry) and I didn't run any type of analysis.

There is something in that chassis that I'm going to take a further look at as soon as I copy the photo down. Film at 11.