Hi Air Engineer, I think it all depends upon what you are flying and at which airfield.
If for instance you are flying a Husky from Doncaster, you have enough room for... A Take-Off,,, Practice EFATO,,, Touch and Go,,, and still have runway left over.
The PAPIs are mostly aligned for CAT landing well into the runway, and most GA aircraft want to be on the ground well before that.
Gliders in particular, practice landing as close to the Threshold Hedge as possible as they are often faced with land-outs in some very small fields. The aiming point is often in the field before the landing field, with the flare occurring just over the hedge.
The geometric PAPI 3 degree glideslope obviously terminates at the PAPI installation. For a CAT airplane the height of the Pilot's eye-line is many feet above his wheels, so the wheels can touch down many metres short of the PAPIs. That is why the PAPIs are several hundred metres up the runway from the threshold.