Balesir
Adventurer
You are right and they are wrong; I have been running for a group of seven players where the party has no "leader" class characters - it works fine (with 3 defenders, 3 strikers and 1 controller, FWIW).4e produces the same results if you build your character based on what is fun to play rather than what the party "needs". For example, I asked the players in our group to skip playing a leader (I had played a leader previously) but I was unable to convince them that it wasn't "mandatory" to optimize the party.
I don't know about "the wrong way" - the roles are a way to get specific and differentiated "best at" tasks for classes, and defenders actually having ways to defend is a big boost to interesting tactics in my experience.Teamwork is very important but 4e roles do it the wrong way.
The roles are not the only - or even the most significant - way that 4E enhances and encourages in-combat teamwork, however. The importance of positioning and the synergies between powers are at least as important, I think.