Happens quite a lot particularly north of Tollerton, especially near Syerston.
imagine the scene. You call a radar unit and are given a squawk then identified. On the screen there are 15 contacts in the next 5 miles of your flight, how on earth do you call all of those contacts? And even if you do no one else will be able to get in on the frequency for the next 5 mins meaning that I am providing 1 RIS when I could be providing 10 or more FIS'. In the situation described above the best you can really hope for is a generic warning to "keep a very good lookout multiple contacts ahead." That is generally the use of RIS limited due traffic density.
Hope that makes sense - quiz me more Thursaday if you wish!!