You have greatly misused statistics if you only believe that Willy is marginally better than Nunez or Colabello.
The message that this team shouldn't be sending is giving away a good player in order to play a much worse player in his place. Players understand if a veteran like Willy is traded but if there is no interest in getting something back and there is nobody with a future to replace him then it sends a message that we are trying to make this team as bad as we can but we expect you to play hard.
I think baseball players are a little more accustomed to the business of baseball than that. They understand that Willingham is gone after this year. They understand that the logical move is to trade him for a prospect right now. I sure didn't get any sense of outrage when they made a similar trade last week, shipping Morales for a minor league reliever. Did you?. I think they get it.
That said, I think we might be underrating Willingham's appeal a little. He's hitting .219, but he's got a .360 OBP and a 782 OPS. He's slugging .481 over the last 2 weeks. He's not an all-star, but plenty of teams could find room for him, even in a corner outfield spot.