VASIS was mandated in Australia following several Boeing 727 approach accidents in USA after the 727 first flew. In those days, seniority reigned supreme in US airlines and so older captains who had flown DC6 and similar were automatically in line for the first of the 727 commands.
Accidents soon happened due to difference in propeller aircraft (instant power available if undershooting) and the slow spool up time of early jet engines. Hence the stabilised approach criteria came into being which included engines being spooled up in a jet to the appropriate thrust to ensure acceptable rate of descent on final. Although rudimentary VASIS had been available before advent of jets, it wasn't until Australian DCA experts perfected the T-VASIS did visual approach slope guidance get really accurate. The former red/white VASIS had serious limitations and T-VASIS was designed to overcome those.
Both T-VASIS and PAPI were designed to aid jet transport aircraft on visual glide path which used to be 2.5 degrees in the early Sixties but were later increased to three degrees in Australia would you believe for noise abatement at Sydney for runway 07. There is a long story about that but sufficient to say there were political issues involved with noise complaints under the Sydney 07 ILS. ILS glide slopes were jacked up to three degrees which gave a less noise footprint and for standardisation T-VASIS were jacked up also. I had some practical involvement with DCA in the flight calibration of the jacked up ILS and T-VASIS in 1970.
VASIS was never designed with light aircraft in mind and since these aircraft have a normal landing approach of around five to six degrees, it is obvious that three degrees is far too flat and requires significant power. About the only advantage for light aircraft using visual approach slope guidance is if the aircraft gets ridiculously low (undershooting) and the VASIS/PAPI goes all red. That happens at an angle of around 1.9 degrees which is real flat.
I believe a Thai International Boeing 777 tried that trick recently while attempting to land at night on Melbourne 34. That was enough to give any pilot the shivers.
VASIS/PAPI are not installed purely for glide slope guidance after breaking out visually from a Cat One ILS. They are nothing to do with instrument approaches. Australian regs require jet transports to have glide slope guidance whether electronic (ILS) or a lighting system. Instructors on light aircraft should certainly explain to students how to fly the VASIS/PAPI but only to point out the advantages of these visual aids for high performance aircraft that need a three degree slope as part of stabilised approach criteria.