Indicating should be a habit and not a choice to my mind, along with starting to indicate at an appropriate time and using the right car "body language" in preparing for the manoeuver.
On the receiving side, I don't readily accept that someone will move where their indicating unless I've seen them actually start indicating and slowing for the turn. I've seen too many people blindly leave their indicators flashing for miles on end.
I'd much rather slightly annoy the person behind me through being prudent than blindly accepting an indicator and pulling out into the path of a car that has no intention of slowing. The kind of person who leaves their indicator on isn't likely to be all that observant of their surroundings.