Don't get me wrong, I'm not a superfan but there are reasons for optimism on his outlook. Its not raining turds as far as his projection goes as you seem to think. He is a nice get for two RAGINGLY mediocre pitchers. Not to mention Castillo...
Since 11, Cashner has thrown 755 innings with a 3.65 ERA and 3.65 FIP. That is pretty damn solid. Im not saying Naylor is a dud, but he's not some super prospect either. He's just a decent prospect that is a long ways away.
People just tend to overate prospects. Every single trade that has been made is "What a steal!" by the selling team. It's not always the case.