He has value to the team, but people are really overestimating his value. He is SF that they can throw at Lebron and Durant, and I think that's his value to the team. He isn't really scoring all that much and his overall defense is not great. There haven't been any games where his "toughness" has pushed the Spurs to a win. People forget that the Spurs won 2 titles after he left and didn't have a "tough guy" on the team.
Point is, he is very tradeable and the Spurs wouldn't miss much, although I don't think they will trade him unless a "too good to pass up" trade comes through the phone. Pop loves him.