D & D is unique in that it is a dedicated service one of the few in the world.
The UK authorities deem that it is acceptable for UK pilots to practice calling D & D in simulated emergencies.
In my 18 years as a PPL I have made one practice call whilst on my own and one position uncertain call a year or two later.
In the real case, I had no hesitation in putting in the call to D & D knowing what the response is like in both style and TONE.
Within 3 minutes I was sorted. I have never had to call again but would not hesitate to do if I had to.
This is in contrast to some ATS units where nothing in the world would make me chose to talk to some of the condesending and smug t*ats who operate them.
Anyway to ensure we retain D & D they need to show they perform a useful service and GA calls underpin the demand. Of course, the airlines do not fund D&D so do not control it like they do with NATS.
I am sure the D & D guys would rather be helping GA pilots and real emergencies that listening to some ignorant foreign pilots discussing who they slept with previous night when they use 121.5 incorrectly.