genesis,
If I am limiting a service im not doing it because its a cop out i'm doing it for a reason. Against transponding a/c only, i'm using an ssr only no primary, limited due to altitude, you are a long way from the radar head and i'm not going to be able to give you much warning from aircraft below or behind . limted due to traffic then there will be a significant number of aircraft, or highly complex situation, not to make things easy on ourselves.
In this way you know where you stand and exactly the service i'm providing. If i say RIS you would expect a certain level of service, if that service is limited for whatever reason then we are both singing from the same hymn book and both know what service to expect.