These are (presumably) Steep approaches, with a screen height of 35ft instead of the normal 50ft. As long as you actually cross threshold at 35ft and Vref, the steeper approach angle will give the shorter landing distance. However; if the approach is flown below the PAPI/PLASI all the way down, the touchdown point would be closer to the threshold, and thus you'd have more runway left when the a/c has slowed to taxi speed.
An approach flown below PAPIPLASI would reduce obstacle clearance and increase the risk of CFIT in case of an unexpected downdraught. At night, the PAPI/PLASI will be almost the sole means of vertical guidance, especially since STOL ports (which I assume this discussion is about?) have a narrower runway and more uneven tarrain surrounding them, thus increasing the chance of optical illusions. Once below PLASI (flashing red)/PAPI(all red), you no longer have a good source of vertical guidance, as the PLASI/PAPI will not be giving any other information than "too low"; not much too low, or "slightly" too low.
Also; if your AA is flatter than the PLASI angle, and you are below PLASI, you could suddenly go from flashing red to flashing white in a split second when passing threshold... This would most certainly increase your lsg.dist.
Our Company has put a lot of focus on maintaining the PLASI/PAPI until passing threshold, but this discussion always pops up come winter and icy runways. Should you risk CFIT prior to touchdown, or hope that you'll be able to execute a perfect flare/touchdown and not slide off the runway?
Not an easy one this - perhaps someone at the tech- or engineer forum could shed some light on the subject?
Cheers!