411A,
It's checks and balances, how many times have controllers had problems when a/c are not flying the SID they are supposed to.
How many times have pilots been issued a SID clearance, correctly read it back and still flown the wrong SID.
Why do pilots who fly the same a/c day in day out, still go through a landing / departure checklist, shouldn't they know it already????
These are all safety issues, it's what makes UK airspace as safe as it is, reduces the RT loading and makes everyones job that little bit easier in our overcrowded skies