This summer, the San Antonio Spurs have been swept up in NBA trade rumors. After lackluster NBA Finals performances from De'Aaron Fox, Luke Kornet, and Keldon Johnson, all three players have been featured in mock trades.

The desire to move on from Fox makes perfect sense. After all, he's expensive and standing in the way of Dylan Harper getting more minutes. Kornet, on the other hand, has been swept up in rumors largely thanks to the Finals. In the regular season, he was perfectly solid.

Johnson, meanwhile, is the longest-tenured player on the roster. He also disappeared in the Finals and is owed $17 million next season. It's the last year of his contract. The Spurs could extend him, but seem willing to let him enter unrestricted free agency next summer. Instead of letting him walk for nothing, some fans would prefer to see him traded.

There's just one problem with that.

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On paper, Johnson should be a prime trade candidate. After all, he isn't on the books for very long and just won Sixth Man of the Year.

Last season, he averaged 13.2 points and 5.4 rebounds off the bench, while shooting 36.3 percent from deep and 51.9 percent from the floor.

Which contender wouldn't want him coming off the bench! Which rebuilding team wouldn't want his experience and contract? If all that were true, why would the Spurs be urged to trade him?

The reality is that volume bench scorers don't command much value. Johnson is limited on defense and isn't a great ball handler, and he can be ice-cold from deep for long stretches.

Look at guys like Jordan Clarkson, Anfernee Simons, and even Bradley Beal. All three of them are great scorers, but that's about it. Clarkson just signed his second veteran minimum with the Knicks, and Simons and Beal are both making around $6 million next season. There simply isn't a market for volume scorers who don't offer much on defense, especially if they lack high-level playmaking.

Johnson's contract is justified in large part by his off-court role. He's a mentor, leader, and locker-room-energy guy. All of that is unique to San Antonio, as those are roles he spent years building. 

His value, for now at least, is the biggest in San Antonio. It's why teams aren't lining up to make a deal.