Roles in 4E basically come in two varieties - Defensive roles, and strikers. All the other roles are focused on keeping the team alive while doing some other fun stuff, while strikers focus on damage alone. Leaders is the most defensive role.
Defense - Leader - Defender - Controller - Striker - Offense
To me, 4E seems based around having about half the party be Strikers. An archetypal five hero party has one of each role and an extra striker. Being more defensive like that creates defensive redundancy. My current party has 2 defenders, one striker (a pretty defensive one), and one leader. I think this is too defensive, which is why the leader is feeling his role doesn't matter. What the group needs and thus what would shine is more offense.
A corollary to this is that the defender is a very strong role, able to both negate and inflict damage. This team routinely faces +3 encounters with no real need of healing or other leader abilities. Thus the leader feels redundant.
This is my best analysis of the current situation in my game.