Even with Blues & Twos on, you'd be surprised just how many drivers still don't see or hear the Police car/Fire engine/Ambulance! There's also the fact that a blind person wouldn't see the lights & a deaf person wouldn't hear the sirens. At least with both on, you stand a fair chance of making yourself known to anyone walking. Same with drunk people - they pay very little attention so anything that can help them not step onto the road has to be a good thing, even if it disturbs sleep for a few seconds/minutes.
I live by Arrowe Park Hospital and I hear the ambulances at all hours but I don't notice them anymore if that makes sense. They don't disturb my sleep now - they did the first 2 weeks
As Ludwigvan states, there's soon be a right hoohah if something happened to a MoP because they weren't on.