*Opens book and reads aloud*
In the beginning, Frank Wren created a third base platoon. And third base was no longer waste and void. Wren said, "Let Juan Francisco play third base against right-handed pitchers, and let Chris Johnson play third base against left-handed pitchers." And Wren saw the situation, and it was good. He named this the platoon.
One day, Francisco and Johnson were walking through Turner Field, and they came across a special door they had never seen before. It said, "Excessive Strikeouts and General Ineptitude". Johnson told Fat Juan not to go through the door because Wren had told them not to, but Fat Juan disobeyed, not believing Wren would do anything. When he walked through the door, Wren was waiting on the other side, and as punishment for Fat Juan's transgression, Wren designated him for assignment and banished him to the Dead Last Sea in the NL Central.
Now, what have we learned from this, children? Well, the platoon was a well-thought out plan. It combined the strengths of Francisco - hitting righties and better defense - and Chris Johnson - hitting in general - while masking their overall weaknesses - hitting lefties and defense, respectively. As things often go, this plan didn't go according to ... plan. Francisco wasn't really "bad", per se, but with a roster crunch coming and Johnson hitting well, the Braves decided to DFA and eventually trade Francisco, making Johnson the primary third baseman. I'm still not a fan of the move, but seeing as it's done, we have to begin looking at the situation as it is now. And the question becomes "What do the Braves have in Chris Johnson?".
Johnson's virtues and sins are easily recognizable. He extols the virtue of above-average offense while sinning frequently on defense and on the bases. Even considering his .416 BABIP (Batting Average on Balls In Play) that no man on the planet could hope to sustain, Johnson remains an above-average hitter, hitting .286/.326/.440 for his career with a wRC+ of 106 (a way to weigh OBP, ISO, and parks played in; 100 being average). By hitting an abnormally high number of line drives for a major-league hitter - 25% against the average of around 17-18% - Johnson can maintain a fairly decent batting average despite striking out in almost a quarter of his at-bats. While he doesn't walk a ton, he walks enough to maintain a slightly above-average OBP, and he adds enough power to be a fairly valuable offensive player. It's not always pretty, but it gets the job done.
But he has his vices, namely defense and baserunning. Among all third basemen who have played 1000+ innings at third base (a little less than a full season) from 2011-2013, Johnson ranks 6th from last in UZR/150, and he has the 6th-worst fielding percentage. One may disagree with advanced defensive metrics, but I doubt most people will disagree with the fact that Johnson has little range and commits a lot of errors to go along with the poor range. He's simply one of the worst defensive third basemen in the majors.
Johnson also doesn't run the bases well. Out of 55 third basemen that have 500 PA in the same timeframe, Johnson is the 15th-worst baserunner according to BsR, and I don't think many of us will disagree that he's generally unable to take extra bases - if you have to run a race to prove you're faster than Gerald Laird, it means you're slow.
Value, however, comes from the sum of these parts, and despite the fact that Johnson is incredibly inept on defense and the base paths, he remains a valuable player because his hitting outweighs those things - it also helps that Andrelton Simmons limits the defensive damage. Hitting, because its potential is unlimited, is more important than defense and baserunning - if possible, you could score an infinite number of runs, but there are a finite number of outs and bases. But how "valuable" is Chris Johnson?
This is where WAR (Wins Above Replacement) comes in handy. While you might argue with some of the numbers, it gives us a general sense of how good a player is in relation to the rest of the league. Johnson has been worth a total of 2.5 wins in ~1600 PA during his career. Looking at it over every 600 PA (or about the number of PA in a season), Johnson is worth about a win per season (2.5 wins x 600 PA / 1600 PA).
Of course, this season has changed that a bit. At this point, he's been worth 1.5 wins, according to FanGraphs, but that does come with some caveats. The first is that .416 BABIP. While that is entirely too high for him to duplicate in the second half, his career BABIP is about .360, so he likely won't come crashing down. Johnson is also in his peak as he's 28 years old, so having an improvement in his performance is also possible. There's a chance that he's better than he used to be - he's walking about the same as always and hitting for similar power, but he is striking out less, which will improve his BA and OBP. But we shouldn't expect another 1.5 win half from him.
What should we expect? Something closer to his career numbers. Again, he's been worth about a win per season so far, so if we split that in half, he should be worth about a half-win for the rest of the season and maybe 1-2 wins for each of the next couple seasons if he maintains peak production. That's not terrible. Many teams have replacement-level players or worse occupying spots for them, and as long as Johnson hits, he'll have value. That's not bad for a perceived afterthought in the Justin Upton trade.
So why are people talking about trading for a new third baseman? Because there aren't many glaring holes on this roster. While the Braves could use some relief help, that shouldn't cost a lot to add, and while an ace-type starter would be nice, the rotation isn't currently a pressing need. Third base is the only real option on the roster where the Braves could feasibly add a better player. The problem, of course, is that there aren't many options currently on the trade market that make sense, so the question is likely moot for the rest of the season.
But what about beyond? Johnson is a Super-Two player in his first arbitration season. He is making $2.3M, and if he hits .300 - arbitrators heavily favor more traditional stats - he'll likely make around $4M next season. For 1-2 wins of production, that's not too tall of a price, but the price will only go up from there. The Braves could try to wrangle a David Freese (not much better than CJ), Mike Olt, Nick Castellanos (would have to move him back to 3B after being in OF), Will Middlebrooks, or maybe even Chase Headley (would be difficult given current perception of farm system) from their current team. There are no real options on the free-agent market unless you love Kevin Youkilis or Mark Reynolds. They could try to trade for a player and move him to third base, but that would be somewhat of a surprise move.
So the Braves are likely stuck with Chris Johnson for the foreseeable future. But that's not a necessarily a bad thing. As it stands, he's a decent player who is relatively cheap, and he's in his peak years, making a collapse less-likely than if he was older. It's not easy to find guys who give a team positive value. If a situation presents itself that would provide a better third base option, I would expect the Braves to take a look, but given that the situation looks pretty muddled on that front, the Braves at least have a seat in the third base game of musical chairs. It might not be a pretty seat, but it beats being left standing.