It's stupid not to
At least with my AT&T plan, I get the $200 price every 15 months. I upgraded my 4 to a 4s and sold the 4 on eBay for $350. How can you beat an upgrade that pays you $150 to do? Even if the phone wasn't upgraded, it would be worth it for a new battery! There's tons of people on eBay looking for recent models and I've sold a couple of 4 s within minutes of posting. Even better now is that AT&T will unlock the old phones which makes them more attractive to T-Mobile and non-U.S. buyers.