It could have to do with the lead in to the turn.
LOC capture gains are such that the capture will be quite aggressive, as if outside the beam it was full of "Mordor like" mountains and obstacles.
In HDG the airplane will wait longer until it starts turning than in NAV or LNAV. If you are fast it will probably overshoot once and then recapture also aggressively and then maybe overshoot again once before capturing accurately.
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.
In VMC or well above MSA I favor a NAV, gentle capture when I fly fast, rather than a HDG capture.
Last edited by Microburst2002; 18th May 2012 at 10:45.