I am intrigued by the idea as well.
Yes, it is not system messages of the wikibase extension, it is system messages of a wikibase instance as Denny correctly explains. But then, sharing theses translations in a central place would tremendously help those who need to set up their own wikibase installation. I believe in the longer perspective, a large percentage of the properties (the descriptive vocabularly of "field names" in a database sense) that separate wikibase installations would use, would consist of re-use.
Whether the re-use mechanism involves translatewiki, or whether it should become a function of wikibase is an open question. I think not now, not while resources for the wikidata project are scarce, but I think it should be conceptually on the map.
--gregor
Can actually translatewiki be merged into Wikidata?
Cheers Yaroslav