Gary Walker's value is not just based on physical presence. He has leadership ability and experience. Not to mention he was pro-bowl player. I am sure he wants to healthy.
I'm sure he wants to be healthy too, just like everyone wants him to be. You are also correct that he was a Pro Bowl player in 2002. This is 2004 and I'm talking about 2004, 2005, and 2006...
Being an experienced leader only goes so far when you can't get on the field. They don't pay guys top NFL salaries to stand on the sideline and clap and offer encouragement to younger players. They need 2 or 3 high quality years out of the guy (on the field -- 30 to 40 games worth) to get the value out of that contract.