Based on the drafting of Collins and Beauharnais, it would appear that Belichick is looking to adjust the LB spot a bit. Certainly this may have to do with Spikes's contract expiring soon, and the need to replace him. Not to mention that Collins is a versatile, do-it-all player. But Beauharnais is more of a prototypical LB. He can hit, run, and appears to be smart and has a nose for the ball. Needless to say, he was a good pick up.
All that said, who exactly on the defense isn't working out as Belichick wants? As we know, Belichick was caught saying, "these guys just don't get it" at some point last year, when referring to the defense. He's clearly looking for tough, unselfish, INTELLIGENT players on the defensive side of the ball, and that's why he brought in 4 RU guys.
I find it hard to believe it could possibly be Mayo or Hightower. Ninkovich and Jones seem like unlikely candidates, and of course Vince and McCourty are consistently lauded for their play and intelligence. The only guys who jump out to me would be Spikes, and their interior DL, including Love, Deaderick, etc. But I would've thought they would've picked up an interior DL if this was indeed an issue, which they didn't. So to me, Spikes seems like the only logical candidate. Nonetheless, we'd be absurd to pretend he doesn't have some value on the team.
Am I overanalyzing the situation, or are there a few players Belichick feel are dunces who can't understand their system?