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What He's Done
Jarret Johnson is, by all accounts, a total badass. He's strong, mean and loves to hit people. Last year was JJ's first with the Chargers after a long and bright career with the Ravens. For last year's Bolts he started 15 games, collecting 40 tackles with only 13 assists, and contributed 1.5 sacks (0.5 more than "pass rusher of the future," Melvin Ingram).
According to Football Outsiders, Johnson had an excellent "stop rate" of 85% against the run in 2012, with an average of 1.7 yards. It doesn't take a math genius to figure out those numbers are very, very, good. Although Jarret did not play enough snaps to be considered in their overall rankings, he was simply terrific against the run by stats alone. Jarret was brought here to set the edge and stop the run and, boy, did he do that.
Why He's Here
Stopping the run is still a thing, and Jarret Johnson can hold the damn edge against the run. This is because he's really strong and yet not very explosive. So, sometimes his "pass rush" looks more like he's simply reading run and is trying to hold the edge again.
Unfortunately, he's not very fast and is only associated with the various conjugations of the word "explosion" during sentences like "Tony Moeaki showcased his explosive ability, bursting past Jarret Johnson to grab the pass."
Playing in the same division as Peyton Manning and the Andy Reid/Alex Smith Gila Monster Pu-Pu-platter, getting after the quarterback is a much higher priority than holding stout against the run. Jarret just doesn't really contribute there, and will probably be taken off the field in favor of a "superior" pass rusher.
Still, as the starting linebacker on a unit that is paper-thin, Jarret Johnson is an important part of the San Diego Chargers run defense.
Why He's Not Higher
Jarret's not a very exciting player and he's not getting any younger. If Jarret has a GREAT season, by his own standards, he'll end up somewhere between 28-40 tackles and maybe 3 sacks.
You won't see much impact from him on the stat sheet and what Jarret does well (hold the edge against the run) doesn't win games. He's basically a luxury player that can fit in really well for an already great team.