Paris mapper Pieren has written an excellent half angry, half regretful diary entry that sums up why unthinking imports are such a curse to OSM. For those that don't speak French, I'm posting a (slightly loose) translation here. Apologies for any errors of translation.
I sadly discovered today that the import of buildings in the 16th arrondissement [district of Paris] has been brutally completed by someone else, by means of the simple import of a file.
I'd been working on this area for several weeks and yes, that might seem too slow for some. But if I was progressing slowly, that’s because I was taking the time to check names and one-way streets on the ground, block by block; to clean up errors (disappeared names and POIs); to add extra information such as traffic lights, missing vélib [cycle hire] points, missing names, barriers, other POIs, addresses. Everything that a semi-automatic import doesn’t do (after all, I could have done the import myself in 15 minutes long ago).
I've already carried out this work for other arrondissements (smaller ones, true) over a long period. You only have to read my OSM diary to see the list. I was three-quarters of the way through working on this arrondissement, but here I'm stopping, disgusted. I shall leave the work of completing the missing information to others, so that the arrondissement may be consistent in a far future (if ever), as well as the other arrondissements of Paris.
I understand that many people work on each area, and it’s better like that. But a tiny bit of co-ordination and investigation, to check what others were doing, would seem to me to be a minimum before embarking on such an import.