earnest,
As has been said before, the route is mandatory because there have been case of planes not flying what we have on the strip. An example of this would be XXX who thought they were going to fly down UN864 (MONTY - BCN - BHD) but we thought they would fly UN862 (NOKIN - CUMRI - TUTON - BHD). When we started giving them their own nav back, they didn't have the points in the FMS and it could have caused problems for different sectors if they had flown on their plan and not what was one the strips.
As an aside, if an aircraft strays too far from it's NAS flightplan route, NAS no longer processes the coordination for us and we get out of association grey strips.
CF