I don't think there's anything wrong with Keyshawn as a backup or even a #3. Look at his game logs from last year. There were only 3 games last year where he scored less than 4 points. Granted, he scored 5 points four times, but he also got eight points or more 7 times. How many WR's that you expect to go for TD's bust every week for 30 yards and no scores?
You aren't getting him for TD potential; you're getting him for the steady point stream. Those type of guys can be important to a fantasy team, especially when you pick WR's that don't get you the TD's you're looking for.
Keyshawn is a good possession receiver, and he gets decent yardage almost every time. In 10/6 settings, he tied for 27th in WR scoring last year. In a 12 team league, that's a #3.