Are we talking about non linear loads in the 3 phase multiwire? That isn't good for the nuetral, but a purely resistive load shouldn't have the harmonic problems. I don't see this being an issue on a single phase service,which is a multiwire circuit by its very nature. I use multiwire circuits in rewires. Pulling 3-wire into places too small for two 12-2's in quite a labor saver.
