[Kevin Ainsworth]

The main reason for the fixture is to save time.

Though, when you cut and join the tubes precisely, allow them to butt up against one another, the load in not 100% on the rivet and gusset. The load can be distributed directly from one tube to the other. This makes the fastening stronger and the complete assembly more rigid. Especially if you are using .04" wall tubing which has a lot of deflection under load when only using rivets.

We have six Bridgeport/Prototrak mills.
There are times where we "tie up" all six but there is usually at least one open. The actual cutting takes the same amount of time as any other method. The setup time is minimal when you look at the overall time savings. We used to hand measure, mark lines with a square or a protractor and then manually cut the tubing. When you have 50 pieces to cut the fixture is a big time saver. Plus the tubes fit together the first time.