The only player that should be on the list of "Good players that improved after leaving the club" is Rooney. Arteta is woeful for Arsenal at the moment, Pienaar's been sat on the Tottenham bench for a year and Yakubu's stuck at Blackburn playing an 8-1-1 formation, just to name a few. All three of which were up there with the best players in the league on their day when at Everton.
You've also got to remember that it's easy to look good in a poor side, e.g Charlie Adam at Blackpool. Since he's moved to The ****e he's looks preeeetty average (although, they are fairly gash too).
Also, even if we did keep Ruddy, I doubt he'd have got a fair shout at the number 1 spot as Moyes is fairly set on Howard being here for while longer yet. I'd rather see Ruddy go somewhere else and make a name for himself than sit on the bench collecting a pay-check.