I wouldn’t say it’s forbidden to re-use building areas. It’s relatively common to see shops mapped that way, so it’s at the very least tolerated. And if you want to continue using that mapping style, it’s unlikely to cause much conflict. (Depends on the opinions of your fellow local mappers, of course, but still.)
At the same time, I’d argue that a separate area is clearly the better representation, semantically. So while I wouldn’t object to someone re-using a building area for shop tags, I would consider it a further improvement to migrate the shop to its own area (which you seem to agree with given your scale of perfection).
Aside from the issue that shops often only occupy some of a building’s floors, there’s another inevitable problem with using the same way for multiple features: It becomes ambiguous which feature additional tags on the element refer to. Is name=* the name of the building, the shop, or both? Is that Wikipedia link about the historic building or its current occupant? Is start_date when the shop was founded or when the building was built? And so on.
To sum this up, I believe that using the same way for the shop and the building causes some issues, and we might leave this method of mapping behind some day if our quality standards continue to improve. That does not, however, mean that shops mapped using a less than perfect style aren’t a valuable addition to the map today.