Aren't displaced thresholds there due to obstacles in the approach path
Almost exclusively. However, the bit before the displaced threshold can be of a lesser load bearing capability than the declared LDA and shouldn't be used for landing. I'm not talking about light aircraft etc but the big stuff. It may well be possible to avoid the obstacles on, for instance, a visual approach and plonk it down at the beginning of the tarmac but you'd look mighty silly when the wheels sink into the ground and you come to a rapid halt in your own furrows. There are, of course, various regulations talking about ILS glideslopes and not descending below them (as per RAT 5's thoughts) but I'm not getting into the pedantic stuff. Put simply, I don't think (unless in an emergency that requires you to do so) you should land before a displaced threshold, whether you believe it's there for obstacle clearance reasons or not.