Hilariously, if I told you I had a filed an IFR flight plan, FL150, for you, from Goodwood to Prague, but told you nothing of the route, you could get airborne, fly east, call up London Info/ London Control, carry no paperwork at all, and you would be fine - because ATC tell you where they want you, all the way along. One waypoint at a time, usually, sometimes 2 or 3.
And an astonishing number of old-timers do fly this way. They hack together some route, which usually gets rejected by IFPS, then they refile with REROUTE ACCEPTED and once this is in the system, they depart, knowing nothing of the route that is actually in the system.
I think I will spread the word in Swanwick that anybody joining CAS should only be given a clearance to first waypoint then just the phrase "flight planned route" and wait to see what happens...