tic,
Cleared FPL route....about as much use as boobies on a bull! I bet if I said to you "Maintain assigned level." you would readback (in a slightly miffed tone) "Maintain FL340 (as approp) XXX 123."
Experience shows us that errors occur. We the controllers and pilots need to ensure we are all working to the same script.
Certainly, in my world as an acft enters the FIR it is given a route clearance fix by fix for the first couple of sectors. Further route clearances fix by fix are given by the next sectors as detailed in local instructions.
These clearances are issued to all acft, whether a one-off delivery flight transiting the airspace or daily (hourly?) shuttle flights that know the airspace inside out.
I recall once during a very busy period, omitting the route clearance (to a locally based operator) who then followed a different route to the one we had. This (luckily) caused only minor inconvenience to the sectors down route. I, of course, had a very red face for not doing my job properly. Lesson learnt by me - no matter how busy, don't take short-cuts or ignore established procedures, they are there for a reason.
Oh dear!