I've always wondered why he doesn't play more snaps on defense, particularly on early downs. Any theories?
The Occam's razor theory would be that he simply isn't any good at it, while the Patriots homer theory is that he's a secret superstar who only doesn't play because everyone in front of him is even more amazing.
One can be a good special teamer and nothing else. The two best special teams coverage guys in the history of the league are probably Steve Tasker and Slater, and neither of them were worth a damn in any other phase of the game.
I mean, look at Slater. I'm not certain there's a wide receiver in the league better at beating a jam than Slater, who regularly beats off two or even three jammers when he's gunning. Being able to consistently get off the line against the press is more important than foot speed in the NFL and a huge part of what makes the transition from college to pros so difficult for a wide receiver, because the fastest guy in the world can't get open if he's being held up at the line. But, despite being amazing at that and fast to boot, Slater's route-running and hands (which is really hand-eye coordination and body control) aren't at an NFL level.
Ebner probably (no real evidence, but Occam's razor) just isn't any good in coverage. The skillset needed to consistently defend an NFL receiver, tight end, or running back is simply a completely different skillset than the one needed for covering kicks.
Similarly, Darrelle Revis is one of the best cover corners who ever played, and I doubt he would be an Ebner-level kick coverage guy. That's fine.
Think about the list of the greatest return men in history: Devin Hester; Gale Sayers; Billy Johnson; Brian Mitchell; Joshua Cribbs; Deion Sanders; Dante Hall; Mel Gray; Desmond Howard; Rick Upchurch. Aside from Mitchell (who was a passable third down back), Deion, and Gale Sayers (who are HOFers), none of those guys did a damn thing on offense or defense.
I think where Belichick's genius lies is that he recognizes that special teams coverage superstars are extremely valuable players, even if they can't contribute in any other phase, whereas most other coaches would have cut a guy like Slater or Ebner long before they could reveal themselves as superstars because they lack that.