In fact, I just got done running my group through LMoP, all the way to 5th level. And our barbarian didn't kill a single monster the entire time. He moved in and raged, and took half-damage while giving the rest of the party advantage. Which the cleric used to slaughter everything in sight. And our rogue hung out on the fringes and took AoOs to keep all of the mooks in contact. So our barbarian was a "defender" or "leader", our cleric was the "striker", and our rogue the "controller." Did they have to be? No. Nothing in the game mechanics or in their "builds" required them to play that way. On occasion the roles would be switched. That's the strength of 5e. And why the concept of roles is invalid. There are only tactics.