It looks like some of the discussions over at #4015 were ignored while separating link and post categories. There are certain sites where this separation causes unnecessary duplication of categories and the headaches that go with it.
Some of these sites, including mine, could benefit if WordPress provided an option to use one category structure for both links and posts. With the current taxonomy architecture, this could be achieved if an additional entry is made in the term_taxonomy table, setting a category as both category and link_category.