I may have been wrong yesterday. I have reconsidered my opinion for no more than 60 minutes, and restate it herewith:
Situation: the Nats IF is good enough defensively, but the team needs MOAR RUNZ
Look, I know that the Nationals SPAD (speed, pitching, and defense) philosophy calls for ground-ball pitchers and tight infield defense. Fact is, grounders don't tend to go over the wall or for extra bases, so we can let a few sneak through the infield in the name of getting a bit more oomph at the plate.
I'm not going to appeal to the advanced defensive stats, because this is clearly a case where our own lying eyes know best: Kevin Frandsen is a +15 run defender at 2B this season, but Danny Espinosa is -5? The real threat is fly balls in the outfield, especially given the tendencies of the Nats bullpen: Rafael Soriano and Tyler Clippard both have fly ball rates near or north of 50%. We can afford to give on IF defense if the OF is solid.
Solution: Zimmerman back to 3B, Rendon to 2B, Espinosa to bench
Now, why would give on IF defense? To get more runs! The Nats are roughly in the middle of the league in most offense stats. Where do the big bats live on the Nats? In the infield! Adam LaRoche owns a 156 wRC+, while Anthony Rendon has a 110, and Ryan Zimmerman has an eye-watering 199! Ian Desmond's wRC+ is only a league-average 99, but I'll cut him slack for his team-leading 11 dingers (plus, I'll accept a slight compromise in offensive output to get his range at shortstop).
Danny Espinosa, on the other hand, sports a Kevin Frandsen-esque 74 wRC+. Trading him with Rendon is probably even between the shift in defensive adjustment from moving Rendon from third to second and the falloff in defense between Tony and Danny at the keystone. Effectively, the switch replaces Espinosa's projected 7 runs below average on offense for the rest of the season with Zimmy's 11 runs above average. That, my friends, is a swing of almost two full wins. Remind me how many games back we are right now? Case closed!