Then suddenly the REP prefix became much more useful than it used to be (around Nehalem I think?) to the point that REP MOV could be as fast or faster than SIMDified copy routine.
REPNE SCASB is useful to implement strlen, and REPE CMPSB for strcmp. Don't know offhand how optimized these are though.
REP LODSB, on the other hand, is completely useless :)