Think of reference markers not as mileposts but rather as lat/long coordinates with a different system, giving a unique identifier to a specific physical location. If the record changes, then everything tied to that record is no longer tied to that actual physical location. This is important when you consider that a "database" in state government often isn't a centralized system like one would assume, but could actually mean a collection of spreadsheets, word documents, and scanned PDFs in a network folder.
The biggest issue is crash reporting. When an incident happens on the state highway system, they way the location is recorded is by reference marker. Thus, if the marker changes or moves, someone who needs to know where that incident happened will get incorrect information. IIRC, the responsibility for who overall maintains the reference marker system (in the office) has shifted a couple times, as information has migrated over to GIS systems that no longer need the makers. Heck, many people need to be fluent in multiple systems - route/milepoint and reference markers - as different groups use different systems.