<<NEVER in 43 years of aviation has an ATCO TOLD me when to turn off the runway, and I hope none ever do.>
Guess you never operated into a busy airport, BOAC! During my meagre 20+ years in the tower at Heathrow I frequently specified precisely where I wanted an aircraft to leave the runway. With parallel taxiways adjacent to the runways it was often necessary to avoid bangs.
I don't know what the College teaches nowadays but when I was brung up, the guy in the tower was responsble for separating traffic and providing a good service to aircraft. If that involved specifying where aircraft should leave a runway then that's what he should do... and he should have enough training to ensure that he knows a bit about aircraft performance.