Frankly speaking I felt uncomfortable that all OSM 3D and OSM 4D tags were supposed to be kept in the main OSM database. In my opinion OSM in its current state is a pure 2D map. My vision was to have a separate server to keep 3D and other stuff (e.g. a hundred years old maps).
It’s an interesting plugin (I had somehow missed it until now), but I do not think that storing tags in an external database and linking them against an OSM object ID would work particularly well. It might lead, for example, to mappers removing data, e.g. when re-drawing a building outline, without being able to notice this.
But more generally, are some additional tags really an obstacle to editing a building for those not interested in 3D mapping? I wouldn’t have considered this a major issue. If anything, the building part geometries may increase complexity somewhat, but the plugin apparently does not help with that.
Generally is the great idea: “keep it simple” of course great idea. And I like it.
I was in some standardization committees. And I know: it is terrible difficult to explain all participants all consequences of decisions in specific areas.
I try in next days to collect some pros and cons for the Idea of separate 3D map.