The FMM is mainly used to tie-in with the 180-day Temporary Vehicle Import permits. If your migracion permit is expired so is your car permit. If a person is caught with an expired FMM driving an RV with an unexpired 10-year permit, then that permit is void. It's all about enforcement and interpretation. Yeah I checked this out with both INM and Aduana and it is fact.Mexico has some wacky laws and officials enforce them (or not) the way they see fit.
When FMMs become computerized they will use scanning mechanisms. Can't you see "Modern Mexico where a patrol car passes, scans your license plate, then the data is processed identifying the driver, his FMM, The car permit and any outstanding citations? Enough to scare a person out of a bad dream...