Locking doors on R44
I had a few of those in the early days, especially when heli was not level before take off and pax failed to get all 3 locks in.
With experienced people on board I agree with the slowing down, and yawing part, but then again with experienced people this should not happen to begin with.
With unexperience pax I would not do that, 2 out of 3 locks is more than enough.
I always formally check wether all door are fully locked (3 pins) and in preflight if the hindge securities are OK.
When people take out doors for photos etc, they might not put the pins on the door, so doing the arcrobatic moves with the doors may lead to dropping/loosing the door, this may be more serious than loosing a Mars bar.