Seems rather odd to call Christiana Aguilera's voice her instrument, and I've never seen it done before. Oh, well.
I would say that in order to have someone's voice be called their instrument, they have to have some appreciable talent ;)
At best, the things tend to be redundant, by simply duplicating what's already in the article. Sure, they sometimes act as a summary, but often too much gets compressed. One infobox described Beethoven's genre as "Classical, Romantic". I know that brevity is a virtue, but surely that's going too far!
I think perhaps brief genres are the result of inattention to the article by an expert on the subject matter. I certainly don't know enough about Beethoven to describe his genre in any more detail than that and would pass by that statement without a second glance.