I don't recommend it, if for nothing but the reason of "keep away." I know you shouldn't draft with an intent to trade, but the backup is key for injury, poor performance, bye weeks. And if you get a good one, you also have trade capital.
One thing with mcnabb is, you know feeley isn't going to be rostered, and if he goes down midseason, its doubtful someone will pick him up just to spite you, because of roster limitations. If you think the system is good, his real-life backup can be worthwhile. Garcia was a better than comepetant backup, and I actually think feeley is the better player.
I'd probably refrain from doing it for manning though, as if he goes down, you pretty much will lose because of whom you had to turn down to get him.
The opening scene of the movie "Saving Private Ryan" is loosely based on games of dodgeball Brian Dawkins played in second grade.