Commenter John C. said something that caught my eye "I'm trying to think of a team in the NL, possibly in MLB, that has a better bench". Fair point. I remember looking at a fairly awful Nats bench in 2011 and coming away with the thought that, much as John hints at, all benches are pretty bad.
But that was looking at the benches of division opponents - good to know but not the best target. Really, to measure relative quality, we want to know how the Nats bench stacks up to playoff opponents. That's who we are comparing them to. The Dodgers, not the Marlins. The Cardinals, not the Mets.
I took a quick look and with multiple caveats* here's what I think
St Louis - Cruz, Ellis, Descalso, Jay, Robinson. Right/Lefty balance in outfield and infield bench. A possible starter in Jay, and Ellis and Descalso both started last year. Stronger defensively than offensively but strong defensively. Catcher back up is weak.
Dodgers - Federowicz, Van Slyke, Gordon, Baxter, Either. Really a two player bench but when one of those players is the 4th guy from Kemp, Puig, Ethier and Crawford that hardly matters. Benches are weighted. First guy should get a lot more at bats than #2, #2 a lot more than #3, etc. etc. Scott Van Slyke looks like he might be a decent fill in at 1st. MI is a mess. Catcher back-up is weak.
Braves - Laird, Doumit, Pena, Constanza, Schafer. The Braves, by necessity, have a decent back-up catcher in Laird (who knows what Gattis will be like) Doumit has potential to be a good bat on the bench and the Braves are less likely to put him in positions where his D will hurt team. Rest of the guys are poor hit, ok field guys. Usual bench junk.
Nats - Solano, Moore, Espinosa, Hairston, McClouth. A poor man's version of the Dodgers bench. OF/1B in ok shape. McClouth could start for a bad team. Hairston might make a decent platoon power option. Rest of the bench is pretty worthless at the plate. Espy is good in field. Moore terrible. Poor balance as only bench we'll see without a straight up 2nd lefty bat.
Reds - Pena, Schumaker, Hannahan, Heisey, Lutz. Heisey is a field first guy but strong enough in that regard to find himself starting here and there. Same to a much lesser degree with Brayan Pena behind the plate. Rest of the bench is garbage. An IF injury would be big trouble.
Pirates - Stewart, Barmes, Harrison, Lambo, Snider. Total defense first bench. Barmes (IF) and Stewart (C) are top notch defenders, but can't hit. Harrison is best bat on the bench and he's not good. OF back-up are weak. Waiting for rookies, hope for their sake that they get here.
I rank the Nats as having the 4th best bench out of these 6 competitors, but more accurately I put them in the 2nd worst grouping. They are a step ahead of the no good PH option Pirates, on par with (though I put them slightly ahead) of the field first Reds, a step behind behind the unimpressive but balanced Braves, two steps behind the Ethier led Dodger bench, and three steps behind the balanced, good field, at least one good PH option, Cardinals bench.
If you're thinking that not taking the minors into account was a big mistake, let's think about that for a second. Who are you most excited for? Souza? You know who has a much more highly touted OF prospect? The Cardinals... and the Pirates... and the Reds. These teams we're looking at are just as good as the Nats and most of them have better farm systems, certainly in regards to position players. If you are ok with the Nats not trying to maximize production when they can because you are expecting them to catch up when their young bats are called up, you are going to be very disappointed. If the Nats are trailing the gaps should grow as these guys get called up not shrink.
*I really roughly estimated WAR totals for this year, didn't take into account what teams actually need, and didn't look deep into splits or minor league situations. This is a scraping look at these benches.