Darko fits the bill too I think. When he became a defensive specialist he was actually very useful, I was sad when his tenure with the Celtics was cut short. I thought he was going to be very helpful to us.
Darko is a different kind of problem. With guys like him, it's an ego/attitude problem, where they can't accept a reduced role, so they end up out of the league altogether. Darko, Keith Bogans, Allen Iverson, Latrell Sprewell, Marbury, etc. (Thought this was going to be Carmelo after the "
Aye P, they said I got to come off the bench" comment and failed Houston experiment, was surprised he held on a few more years).
I imagine guys like Darko in Boston were explained what their role was going to be, and they assured the team they were good with it, but can't deal with it once actually faced with it. Like Iverson in Memphis, he said he was okay coming off the bench, but quit the team because of it after 3 games. It's one thing to say it ("I'm okay with this role"), but it's another thing to watch your team lose while you think you're better than the 2-3 guys ahead of you on the depth chart. These guys always seem to have a very inflated view of their current skill level.
This seems to be a big reason why Dwight Howard isn't in the league right now. And I wonder if this is what has been Cousins' problem lately too. Cousins is a guy who has said the right things to the press (okay coming off the bench, okay with a reduced role), but maybe he's constantly complaining in the locker room about it? (Just a guess as to why a guy who still seems productive hasn't found a home).