Laser tube cutting machine - GIF

[Frank S]

Longitudinal travel would be hampered definitely by stock swaying and/ or dropping as it extends. Depending on length of cut I'd imagine a roller steady rest might alleviate that.

Yes plus there would be the issue of irregular shapes of tubing IE rectangular square cylindrical each would require special treatment for a steady or series of steadies.
With a machine where the tubing merely rotates but otherwise has no linier movement the torch does the majority of the travel through several axis of angularity plus the simple XYZ movements many shapes of materials can be used the steadies can be fitted with cylindrical fixtures which are affixed to the shape. Higher productivity with a much wider scope of work can be preformed even including long flat sheets of metal albeit in somewhat limited width ranges, but that is why you have large gantry mounted burn tables 3 meter by 12 meter plates could be cut on our burn table