The secret weapon contenders should trade for

Billy Hamilton has been in the big leagues for nearly five full seasons now, and it's become clear that he's never going to be even an average Major League hitter. In over 2,500 plate appearances, he's put up a line of .245/.299/.331, which is one of the 10 weakest lines of qualified hitters since 2014, per wRC+. It's worse than 96 percent of hitters with as many plate appearances in the last 50 years. With Hamilton's salary likely to go north of $5 million in arbitration next offseason, he's very possibly a non-tender candidate.
You wouldn't think that a player we've described like that would be someone that contenders would want to go after at the Trade Deadline. But allow us to argue the opposite: Hamilton's breathtaking speed and skill on the bases and in the outfield would make him a fantastic secret weapon down the stretch and in October for more than a few contending teams. Remember how the Royals got value from Jarrod Dyson and Terrance Gore in 2014-15?
We probably don't need to express to you how elite Hamilton's speed is, but we'll do it anyway. His 252 steals since 2014 are the most in the Majors, ahead of Dee Gordon's 237, and according to FanGraphs's "Base Running Runs," which also accounts for taking extra bases, he's first over Mookie Bettsby just a ton.

That elite speed has lent itself to some game-changing defensive skills. Hamilton is tied for seventh in outfield Defensive Runs Saved since 2014. That's backed up by his position in Statcast™'s Outs Above Average metric, which measures range and plays made (excluding arm, currently, for the three years it's available.
Hamilton Outs Above Average rankings in 2016-18
2016: +23, second best (of 208)
2017: +13, eighth best (of 206)
2018: +14, second best (of 174)
Hamilton has been so good on defense and on the bases, in fact, that he's been a net positive despite the total lack of a bat. He has been worth nearly 9 Wins Above Replacement for his career, or about the same as David Freese and Starlin Castro have been worth since 2013, despite a 70 OPS+.

We've maybe not told you anything you don't know yet, because of course Hamilton is fast, a great defender and can't really hit. How would that fit on a contender? And which contender?
Obviously, no team looking to win is going to trade for Hamilton looking to put his bat in the lineup every day. But that's not really the appeal here, either. You don't trade for Hamilton expecting a starter. You trade for him as a late-inning weapon to steal the biggest base possible or improve your defense at the most important time. You trade for Hamilton because for all the things he can't do, he can do things that no one else can.
What that means is that teams with weak outfield defenses, especially in big parks, who could use a defensive and baserunning boost, should be willing to take a gamble here. Again: the point here is not to install Hamilton atop the lineup every day and let him drag down the bats. It's to bring him in as needed, late in games, to make a difference. It doesn't make sense for stacked outfields with good defenders like Milwaukee or Boston, but here's who should do it. And let's also note that Hamilton's name has come up in rumors for years, and the Boston Globe's Nick Cafardo reported recently that the Indians have Hamilton "on their radar," so a trade is not out of the question.
Rockies
This is the best fit, because it's been the best fit for years, simply because the outfield in Coors Field is so massive that it demands a player with elite speed and defensive skills to cover center. Charlie Blackmon, for all of his immense contributions, isn't really that player, and the Rockies are 26th in Outs Above Average at -12. (Most of that is Blackmon, who is -8, as his once-strong speed has declined to roughly average, and he's stolen only five bases this year, after grabbing 43 in 2015.)

Importing Hamilton to cover all that ground off the bench would be a huge win, as it would allow Colorado to push Blackmon to left field and the ineffective Gerardo Parra to the bench, improving two spots. It's also fun to think of what would happen to Hamilton's bat if the huge outfield would turn some of his singles into doubles or triples, though one issue here is that in Raimel Tapia, the Rockies have a version of this type of player in-house. They should do it anyway.
Mariners
Seattle tried to put a light-hitting speed demon in center when they converted Gordon to the outfield. It didn't go well, because Gordon was inexperienced there, and it's irrelevant now, because Robinson Canó's suspension has pushed Gordon back to the infield anyway. Hamilton would take time away from Guillermo Heredia, who is also a weak hitter but without the same elite speed or defense, and wouldn't it be just so much fun to see Hamilton and Gordon both running the bases together? Despite Gordon's presence, this has been a below-average baserunning team.
Indians
Cleveland has Michael Brantley in the outfield … and a near-endless number of questions after that. Tyler Naquin and Lonnie Chisenhall are both hurt, and Tribe center/right fielders have put up the worst hitting production in baseball. (The entire outfield group is also 23rd in Outs Above Average, at -9.) The Indians needed an outfielder for a year. They do now more than ever.
Again, Hamilton won't fix the hitting problems, but the Indians are starting Melky Cabrera and Rajai Davis regularly right now, and they have a division title all but guaranteed already, so this move is about setting a playoff roster. It doesn't hurt that rumors have made this connection already, anyway.
Phillies
Philadelphia almost certainly has the weakest outfield defense in the National League, as the Phillies are 29th in Outs Above Average, ahead of only Baltimore in the Majors. (DRS concurs.) Rhys Hoskins is a first baseman playing left field, and he's often looked like it, which makes this an easy fit: Hamilton plays left late in games, or shifts Odúbel Herrera over from center. But like Colorado, there might be a similar player in-house, the oft-injured Roman Quinn, who has speed nearly equal to Hamilton and was just recalled on Friday.
Others: Giants/Astros/Yankees/A's
Those aren't the only four, but the other teams you'd consider here have less reason to. We know the Giants have been interested in Hamilton before, but a 3-8 slide and the promotion of Steven Duggar might take them out of this. The Astros could use a weapon and some help in left, but Jake Marisnick might be too similar. Would the A's prefer Hamilton over the disappointing Dustin Fowler? What if the Yankees wanted some depth for Aaron Judge's injury, and liked the idea of Hamilton off the bench in the playoffs?
Hamilton probably isn't part of the next good Reds team. He might not even be part of the next Reds team. Hamilton's departure now wouldn't bring back a top prospect return, but it would allow for a longer look at Philip Ervin or Mason Williams over the season's final two months.
For that reason, there's just not that much impact Hamilton can make on Cincinnati. Despite the Reds' recent surge, there are no big games ahead for them in 2018. For a contending team, Hamilton isn't a long-term play. He's someone with a few elite skills, ones that could make all the difference at the most important times. No, Hamilton can't hit, but that's the point. After all, smart teams look past what a player can't do to focus on what he can, perhaps even wildly out-of-the-box ideas. Few can do the things that Hamilton can.