Wow, thank you to everyone who has chimed in!
It is difficult to know what process POWO undergo before they decide to accept or reject a change. I’m not involved in the process and so I can’t say for sure.
In Australia, where I work as a taxonomist, there is a process (APC - Australian Plant Census) to arrive at a consensus taxonomy, or the closest we can get to. Even dealing with a much smaller flora, and with a bunch of herbaria involved, it can take years to assess each taxon or proposed new combination, so I am doubtful that POWO have the resources to critically examine each change.
The rules exist to determine whether a name is validly published, and also which names take nomenclatural priority in the case when a taxon has more than one name. It concerns itself only with nomenclature, in regards to correctness.
I think in the case of GLOVAP the question is more one of taxonomy than of nomenclature. They have re-defined the taxonomic placement of thousands of taxa (moved them from one genus to another). This is a matter of opinion, and not one of whether a name is valid or not. In cases of taxonomy, it is generally usage and take-up of the new proposed classification that eventually decides what name we use for a taxon. Down here, for example, at some point it became obvious that the southern beeches were NOT so closely related to the northern beeches, and needed their own genus (Nothofagus), compared to their original genus (Fagus). There is general acceptance among botanists that this is the case (or even that this doesn’t go far enough but that’s another debate), and so the name Nothofagus cunninghamii is in general usage in Australia, in preference to Fagus cunninghamii, even though (as far as the Code is concerned), both are validly published names.
Many authors have published myriads of re-combinations of names, and many of these made relatively little sense, or add nothing of perceived value to the existing classification. In these cases, the new combinations have not generally been taken up by working taxonomists, and have sunk into relative obscurity, even though they’re often validly published names, and nomenclaturally correct.
Sorry to go into so much detail, but I think it’s important to draw the distinction between two separate issues: cases of validity/correctness, and those like this one, of opinion.