Well, thanks for all the opinons, but noone really answered the question.
The most common reason to displace a threshold is because of obstacles on the approach. If that obstacle is behind you, can you legally touch down on the perfectly maintained, load bearing surface prior to the threshold? If not, where is this LAW stated? If so, where is this law stated?
There are a million reason t want to touch down before a displaced threshold. If a runway is 2500ft long, with a 600ft displaced threshold, and your aircraft requires 1899 ft to stop, that 600 is invaluable.
Many obstacles which are the cause for displaced threshold are off the centerline of the runway, but withing the 5degree cone of approach. As a pilot, it is easy to tell once you've past an obstacle, without bouncing the gear off of it.
Opinions are great, but facts, and quotes are better.