I think this is one of those situations where the pilot thinks the controller works with far more accuracy than he actually does.
You know, even if the turn is a massive right 90 or something, it doesnt make a blind bit of difference whether you turn at the waypoint, a a mile before it, or a mile after it.
Remember, the heading itself is not a GPS track - its a heading! a rough guess, of where the controller would like to see you move along the screen ...
Accuracy in lateral navigation is limited from an ATC point of view - and is naturally so because of radar limitations, wind (which is generally an unknown quantity to us), aircraft characteristics (interia of a 747 turn vs. a 318 for example)
That's why we use vertical separation heaps more than lateral!