I don't understand any of what happened around this guy. WHY did they hold onto him for so long and WHY couldn't he recover? WHY couldn't they see the writing on the wall with him? Would he not drink the Kool-Aid? Was he a loafer? I have little patience for players like this. Same thing with guys like Woods and Alexander. They seem stuck in some kind of purgatory, where they make the roster but somehow aren't solid contributors except for sometimes on special teams. Very frustrating.
My guess, and this is a total shot in the dark, is that he had enough base physical talent that he was worth stashing on IR where he couldn't take a roster spot anyways, but he just wasn't able to develop enough physically to get to a point where he would be any better than a JAG (too frail, skinny, not well-conditioned, who knows).
Basically, I'd guess that he stuck around for so long because there was no real cost to keeping him around. This was just finally the year where Belichick said that that was enough, and he's clearly never going to develop into much of a player, and that's that.
Woods and Alexander, IMO, are in a totally different situation. Their issue is that they just have a low ceiling, and will never be much more than they are. That said, they've reached that ceiling, and they're good enough players to make strong ST contributions and, in Woods' case, provide solid rotational depth at OLB. They'll never be starters, but at least they're worth roster spots. Crable, OTOH, maybe had the raw physical gifts to be a starter, but he didn't have the football skills to even make the roster.