D&D is a great service in the UK and of course 121.5 is well monitored by a range of entities, including airliners. However, the overall point of a number of posters (including myself), is that having already established two way contact with an agency and having your radio tuned to them, in most cases, you will be better off broadcasting your initial Mayday to the agency you are already speaking with rather than spending time and mental bandwidth switching to another box, or dialling up 121.5 (even if it is a simple button push). The specific case I would switch to D&D (but not 121.5 generally) is if I was not talking with a radar unit and was unsure of (or unable to articulate) my position and had reason to believe I might not be in SSR coverage, when D&D's VDF fixing capability would be specifically very useful.
Of course if you just getting a basic service from an aerodrome 10 miles away (why?), or not talking to anyone in particular, 121.5 is the obvious choice.
There is also the London Information case, where people indicate they call up, get a service, and in the specific case motivating the thread are expecting London to come back with a new squawk and handover for Lille, etc.; but the frequency is too busy to actually get a mayday out - a case I fundamentally don't believe exists in any material way, as if it is so busy you can't get a Mayday out, why would you try and get a routine service from them - hence you would choose to be in contact with someone else or no one at all (and then clearly 121.5 is your goto frequency).