All sorts of things work for the outer rollers, a single axis. The problems start when you get all three axis intersecting. On the middle assembly, if you did that slot method and did not have all 6 slots in the same exact spot your z axis would not be 90 degrees from the work surface. It might not be drastic, or it might not be that big of a deal but I would rather come up with a solution that does not need 6 bolts to be adjusted the same.
Ease of assembly I guess.
The first release of this machine it worked with several sizes. The center, tension bearing, stayed fixed and the wings, bearings without plastic on both sides, flexed open to accommodate EMT variances. This made for a weak machine, and I had to close the center parts to increase rigidity. I can redo it this way with 2 of the 3 bearings fixed and a movable 3rd, but every slightly different size will need a custom tool mount and z motor mount. Not too bad but I’m convinced there is a better way to keep it rigid.