The AIP is unequivocal:"You must at all times maintain a rate of climb or descent of at least 500fpm. If you are unable to do so you must inform ATC. (sic)" It was introduced so that we know that you will continue to perform in a certain way so that we can base separation upon you. When it all goes wrong it gets very nasty. (There was a humdinger of an incident several years back with a 747 for Heathrow and a G4 (I think) for Luton. The 747 called Approach and was told to reduce speed, so he raised the nose and almost levelled off. The G4 which was being descended on top was doing 1000FPM+ and got very close. The controller was criticised for giving the G4 further descent 100ft too early, but realistically it would have all been OK if the 747 hadn't reduced below 500FPM).
Obviously, if you are sidling down the ILS with a ground speed of 85 kts then 500fpm will put you on the ground about 2 miles short of the runway, so the glideslope or other approach criteria will take over! It is also unlikely that there will be anybody descending on top of you in the approach area who needs to rely upon you maintaining 500fpm.
In effect, what the AIP is covering is ROC/ROD in areas other than SIDs or Approaches, which provide their own lateral separation.