If you are creating a good bench you don't have any pieces on it that are one-trick ponies, unless they do that one-trick about better than anyone. Of course that usually only holds for defensive specialists, or speedy players, because if you are a guy who has pop or hits for average about better than anyone, teams will try to find spots for you - even if you do nothing else. (see : Trumbo, Mark)
Stephen Drew can still (probably) field adequately. He can hit with power from the left-side of the plate. His isoSLG last year would have been second amongst all SS and his projected power is around Top 5 for the position. Given those skills he makes a passable back-up MI on the bench (while Espinosa is starting and Turner is in AAA, waiting out his service time... I mean "honing his skills") and a fantastic last MI on the bench (when Turner is up and Espy sits). Yes, yes Drew is a Boras client. And yes yes Drew was drafted by Rizzo when he was in Arizona. Both these things helped get him here I'm sure. But if he's played correctly there really doesn't have to be a downside here.
If played correctly...
So how do you play him incorrectly? Well Dusty, known to favor veteran players, could put Drew in place above Espinosa (better in just about everyway, which tells you where Drew is at). Drew shouldn't be playing everyday, not even as an injury replacement, or a stop-gap. Not when you have a better option and the Nats do. Scarier than that would be the potential logical extension where if Baker puts Drew at SS and the Nats are doing moderately well and he's not god awful, that he stays with him not only over Espy but over Turner as well. Turner is the future and is very likely to hit very well in his "get Nats another FA year" turn in Syracuse. If so, there won't be any reason to keep him down but you can picture that situation arising.
Of course no sense worrying about things that haven't happened yet (Like say the Murphy deal falling through and Drew being pencilled in as the 2nd base starter). I see how this can work out. I'm going to assume that's how it's going to go.
Who does this most effect? Espinosa, certainly. He will at least lose some at bats versus RHP and possibly lose his role entirely. We hope not but the potential is there. Wilmer Difo, thought to have a role in the 2016 Nats, is almost certainly now pushed back into AAA to try to fight the fade into organizational depth (hey! It's the ghost of Tony Renda!). And the aforementioned Tyler Moore may finally, mercifully lose his spot on the bench, seeing as Drew offers the same pop, but can also play in the field as well. Of course Moore does hit from the other side of the plate and today would still make my expected* OD roster (bench : Lobaton, Clint, den Dekker, Drew and Moore)
All in all, this is much like every deal the Nats have made this offseason. It makes them a little bit better. That all adds up. Of course they got worse to start FA and still have a couple moves to make that will do more of that, at least on the field, so we still have to hold back any evaluations until later.
*not to be confused with my preferred OD roster.