The reason team's don't have productive hitters up and down the lineup is that such hitters are scarce and teams can't afford to do that, not because there's a need for certain "roles" to be filled. An All-Star lineup, like, say... C-Soto, 1B-Pujols, 2B-Utley, SS-Ramirez, 3B-ARod, LF-Braun, CF-Hamilton, RF-Dunn... would easily score the most runs in the league and, given non-terrible pitching, would win a ton of games. There's no need for role-players. They're just there because a) great players are rare and b) teams can't afford to have great players at every position (the two go hand in hand, obviously).