Nobody used "LOOP", for example, so it tended to be poorly optimized, which made compilers even less likely to use it-- just use the seperate instructions that have similar effects. Eventually it actually became a liability to run it efficiently. (The famous "Windows 95 won't work right on a fast K6-2" bug is based on this problem-- LOOP was much less efficient on a Pentium than a K6)
Wow that's pretty interesting on it's own. It turns out a program was trying to figure out long the LOOP instruction would take to run by running it 2^20 times and dividing it by the amount of time taken in milliseconds. On a Pentium 3 this took 17ms, but on a K-2 (and faster) this could take 0 milliseconds.
https://gigazine.net/gsc_news/en/20200606-windows95-failed-s...
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 :)