Hmm? Well let's see. What is it we need mass for? We need a
proportionality constant between momentum and velocity, and we need
something to be proportional to the total energy of a particle.
In neither case will the rest mass suffice so we are sort of stuck with
the relativistic mass. That said, I must admit that the concept of the
rest mass is useful in the theory in an way that is analogous to the
usefulness of the proper time. WBN