Microburst 2002:
In NAV or LNAV mode the turn initiation will be speed sensitive, so if you are really fast it will start turning very early, maybe even with small bank angles and capture without overshoot, and somehow that is not desirable because it would be dangerous in that world of killer obstacles and terrain outside of the ILS beam. That's what I think.
The protected airspace is considerably wider than the ILS "beam."