Yes this is why.
I have had customers use supported (TBR) rail for Z, and it worked, but i dont suggest it.
If the Z is a short stroke, standard 16mm should be sufficient, for light wood and plastic, or 20mm if you can stretch to it.
I dont suggest using 12, 13 or 16 mm rail for X and Y, unless doing very light work or engraving.

Quote Originally Posted by irving2008 View Post
Supported for X and Y is preferable because the weight of the load (router + vertical forces) causes significant negative deflection in the vertical plane which the support effectively resists. However supported rail is strong to resist loads into the support but poor when being pulled away from the support which is why Gary recommends something stronger such solid profiled rails where the glides run on the edge and better resist forces equally into and out of the plane of the rail as well as turning moments.

Thats doesnt mean you cant use unsupported rail in thr Z direction. In reality the deflection will be much smaller than what I suggested and for a small machine 12 or 16mm rail may well be adequate if the depth of cut is kept small enough. As Lee says, his works fine...