Are long brines neccesary for sous vide? I'm looking at a recipe which calls for two weeks of wet brining followed by 48 hours of sous vide for beef tongue. I've seen similar recipes for other tough cuts of meat such as brisket.

It seems to me that if we're going to stick it in a vacuum, the meat can't really dry out that much in any case, so I'm not clear on what the brine, especially such a long one is accomplishing.

What am I missing? Are long brines for tough cuts really necessary if you're going to sous vide them afterwards?
Typically brines are for tenderizing and penetrating flavour into the meat. 

The use of the brine would be to break down the tough meat

Though a sous vide does tenderize mildly with the long cooking time you stated, you don't want to over cook it, hence the brine.

Nor would the sous vide alone add as much flavour as the 2 week brine.

The brine is more for a tenderization than to prevent drying out.