DFC.. I hear what you're saying but there is the factor of anticipation to be taken into account. A pilot watching for a particular point at which to commence a turn is ready to turn at a precise instant. When he receives an ATC instruction he has to absorb the instruction and then take the necessary action, which may take several seconds. The controller automatically - through training and experience - takes into account the time it takes for him to transmit the instruction, for the pilot to react and for the turn to commence. Believe me, it would be an exceptional challenge to get a large aircraft doing 180 kts on a 90 degree base leg on to a centreline from 1.8 miles away. A clockwork mouse might be different, but not a 747-400.
On the rare occasions when we were able to let pilots do a DIY ILS it amazed us how close to the centreline you could get and still complete a succesful intercept. On more than one occasion I've commented that if I'd left the turn that late they'd have screamed through the localiser!
Any current controllers care to comment, given that I'm long out of practice and have to rely on a beaten up old memory box?