Try this for size:
Not all days are worse than CAT 1, and if you become visual by say 500ft, you may well disconnect the automatics and fly manually. The PAPIs provide visual guidance towards the runway.
On those days which pea soupers, then PAPIs are irrelevant, as the automatics are still in flying the aircraft on the electronic glidepath to touchdown.
Not all runways are equipped with ILS, so the minimum RVR will be much higher for say a VOR approach.
Also think about a circling approach, and how useful PAPIs can be in these situations.
The bottom line is that the meteorological conditions are not constant.