I suppose the higher allowed load is because of better heat transfer. If you put it in a confined space, or in a harness together with other cables, the heat wouldn't have any place to go anyway.
And, unless the thinwalls are superconductors and not copper, the voltage drop would be the same on same square cable; and that's often the problem in low voltage applications (i.e. car battery voltage). Also think of physical/mechanical strenght, but that might very well be taken care of with a stronger type of insulation.