Currently, non-sysops cannot upload images to a local wiki if that image is
already uploaded on Commons. This is good. We should, however, do the same thing
for the image descriptions. That is, there is no reason for, say, the English
Wikipedians to edit the local image description of an image that is stored on
Commons. The whole idea of Commons is to be multilingual and the free image HQ
for all wikis, and local descriptions collide these goals.
Also worth considering is that Image description pages, especially for those on
Commons, are very loosely watched, and are many times used to vandalize or by
unexperienced editors who think that's the place to discuss the image. All edits
to a description of an image on Commons should logically be made on Commons.