Indeed. ​There are three use cases:

1) user looks at image on Commons, wants to reuse it on some other wiki

2) user looks at image on some wiki, wants to reuse it there in a different article

3) user looks at image on some wiki, wants to reuse it on another wiki

Showing the namespace in the content language would break 3. Showing it in English would inconvenience 2. Using the subst trick has the optimal result in terms of saved wikitext (assuming all communities prefer localized namespace prefixes to English ones) but looks scary.

3 would typically happen when someone browses the interwiki links to find more content for an article. 2 is probably more frequent on large wikis which have properly categorized local uploads.

The same question could be asked about the "thumb" keyword in the wikicode of the thumbnail, although that one probably does not get translated as often.