"Master Chief Petty Officer is the ninth, and highest, enlisted rate (paygrade E-9) in the U.S. Navy and U.S. Coast Guard, just above Senior Chief Petty Officer. They are referred to as Master Chief in most circumstances. They constitute the top 1% of the enlisted members of the maritime forces."
It's actually a truth that many Non-Commissioned Officers do not start over as lowest officer. (why would they: chewouts are bigger, supreme reduction in authority, time-in-grade pay for 20+ years is niiiice.) Officers "command", but the NCO's run things. My brother is US Navy Captain, and his whole career he can get NOTHING done without "the Chiefs") For some perspective, Sgt. Major Johnson and MCPO S-117 are equal in rank, but different who's got command authority for ops. And for story: making main Halo character an NCO helps with the "everyman" feel: the grunt who's ranked up, the go-to guy, no long lines, not the heavy cerebral-ness of officers a gamer community might not embrace. (example.. backlash on Noble Team characters.. most are officers.) NCO's eat officers, if they hungry enough.
I like the dialogue in Halo3... Master Chief nearly forgetting to call Admiral Hood "sir". I believe it's not just that he's a Spartan, nor the last Spartan...it's all rolled in one.. spartan + master chief + legendary reputation.