Alternative logic, that still keeps things as simple as possible, would be to have a published figure that assumes you have a TAF QNH. Then, if necessary, you: ...
Point taken FGD. As you say, that would be simpler than the current.
However, I'm a very simple guy. I want a system that defaults to the experience of most pilots, most of the time. And that, of course, is ATIS or equivalent. (Not to forget pilots operating at remote and/or poorly equipped airfields. But take the total number of instrument approaches flown each year in Australia: much more than half are to airfields with approved QNH sources.)
Hence my wish that the default scenario is for an accurate local QNH. When I'm flying somewhere without local QNH (as all do sometimes and some do all the time) I know I have to apply an MDA correction.
If you need local knowledge to interpret the plate MDA correctly, it's less safe than it could be.