This is one of those trades that looks like it will help both sides, but in reality will hurt both teams.
For Philly, you just took a mainstay of the organization and shipped him to a division rival. Every time McNabb wears the gold and burgundy, folks will be reminded on how unceremoniously the best quarterback in Eagles history (there, I said it) was treated. It started with getting booed on draft day when picked, and it ends with a trade to division rivals.
The Redskins are not really helped by this move at all. Sure, you get great inside knowledge of the Eagles, and you get a quarterback with a good understanding of the NFC East, but I don't think that anyone would argue that McNabb is more mobile than Jason Campbell and than offensive line in Washington still has a lot of holes. The only real benefit this will have is when Washington grabs Jimmy Claussen with the fourth pick of the draft, he can now be mentored by a really smart quarterback until he is ready to take the reins. But when you have as many holes as the Redskins and are transitioning to a new defensive scheme, I would probably rather have the second round pick. Frankly, the Eagles had to move McNabb and on draft day, he likely could have been had for cheaper.
-wtk