There are several factors at play here, the biggest being that the Eagles are "rebuilding".
1. All of the teams they have cut the most people, also they are now one of the youngest.
2. McNabb is in his last year of his contract, if you are not going to resign him then you have to get something for him.
3. The part that is hard to figure out why keep Vick? The are tons of cheaper back ups out, you could probably get a Jeff Garcia type of player for the vet min.
4. For Washington this is no brainer, Shanahan will get the most of him.