Aerocat, in my example I was thinking of a place which has not had a regular TAF, but now has one issued. It happens occasionally.
Usually, if you have a TAF, there is no ambiguity. As FGD pointed out above (post 41), a more common problem is when NAIPS cannot provide a TAF for YXXX and you must decide whether that is a one-off or a regular thing. As a commercial pilot, I'm not interested in adding 50' to an MDA without strict necessity.
At the risk of being repetitive, I think a good system is one that defaults to the correct option for most users, most of the time. That would be an MDA based on an accurate local QNH. (I wonder why Jeppesen charts provide exactly that as the primary or default option? Hmm.)
Cheers, O8