This is a smart move if they intend to really focus on just MongoDB. For a company that wants to ship more than a product in the same area to have an "upper level" brand makes more sense IMHO.
The can just name their new products "MongoDB this" and "MongoBD that".
And 10gen wasn't much of a brand name to begin with. Very few people knew it or remembered what it was, whereas tons have heard of "MongoDB".
Initially I had the same thought as the parent post, but this is a good point. No one ever really heard about 10gen; it was always MongoDB. Their events were always Mongo titled (MongoDB Days) and even their emails are @mongodb.com. In a way it was already their primary brand, might as well make it official.