I presented the idea differently to the group than as a balance motivation.
I said "Let's each take a different class and a different race so that we get a feel for how the game works."
This could have resulted in 3 strikers, but it ended up with 2 defenders, 1 controller, 1 leader, and 1 striker.
After a few sessions, one of the players (who is a school teacher and has the entire summer off to design a campaign) decided he wanted to try his hand as DM. I jumped at the chance to play. The player of the controller was not having as much fun, so he switched to a striker and I took a controller.
We now have 1 defender, 1 controller, 1 leader, and 2 strikers.
It has worked out real well, but the group became balanced not because that was our goal, but because we decided to each take a different race and class in order to more easily learn the game system.
If everyone is "forced" to take a different class instead of a different role (via group consensus), it might still result in a group without a leader. But, this is more of a "soft force" than a "hard force" and it will probably result in a reasonably balanced group nonetheless. The group might just be missing a role (or possibly two, but unlikely).