I think another benefit of some of these designs, and a primary driver behind my particular new design (parts go on sale next week!), is the relative ease of setup of these systems compared to round pipe. While there's no rocket science behind bearing on pipe setups, they do require non-90 degree angles, which sometimes makes for difficult manufacturing and alignment setups. I do agree that axial loading probably isn't a real concern, however.
My $0.02,
Ahren
www.cncrouterparts.com