Sorry I'm not sure where you're getting 9% from. The link you provide for `currency:XBT` shows 7.35% with a combination of `building=*`. This key is fairly small in terms of sample size though, only having around 17,000 uses.

Checking some more common POI keys though, I believe this is more common of a combination than you think:

`key` - combination % - ~combination total

`amenity` - 10.98% - ~3.34 Million
https://taginfo.openstreetmap.org/keys/amenity#combinations

This also includes parking, so remove ~70,000 for `amenity=parking` + `building=*`.
https://taginfo.openstreetmap.org/tags/amenity=parking#combinations

`shop` - 19.25% - ~1.28 Million
https://taginfo.openstreetmap.org/keys/shop#combinations

Sure these are still a minority in terms of percentages, but to me over 4 million objects mapped this way means its still a fairly common way to map it.

Like these from around the Greater Houston area:
https://www.openstreetmap.org/way/541169550
https://www.openstreetmap.org/way/261269148
https://www.openstreetmap.org/way/347136369
https://www.openstreetmap.org/way/526884518