One word: corrosion!
RMC directly buried will eventually corrode to the point of radically increasing it's resistance for the path as an EGC. You would then have high risk of creating voltage gradients as well. It would not be wise for either safety or economy to omit the grounding conductor.
Relying on raceway as a grounding path is poor practice anyway.
And welcome to ECN!!
edited typos
[This message has been edited by mxslick (edited 02-06-2006).]
