I think it won't matter what your composition in a fireteam is until a certain point in the game. For strikes (3 man coop), probably won't matter because it will be a quick 30 to 45 min activity with modest loot. It won't matter because most of the strike will be trash mobs and a final boss at the end (just look at the moon trailer). Remember, bungie wants to make this accessible for all players (MMO rpgs AND shooters) so this is where everyone can be what they want to be. For raids, the more I hear about it, the more I'm convinced that they work very similar to MMO raids in terms of difficulty and will only be available late in the game. In MMOs, raids are only allowed to be completed once per week and yield amazing loot if you can get good enough players to help you through the ordeal. They usually are 8-12 hours to complete, which means scheduled play dates are crucial for completion. Bosses alone take 5-10 mins average of non-stop fighting so ammunition/mana must be conserved to last the entire fight. Although with Destiny, it is more shooter oriented so it won't be as long as MMO raids are. However, the fact still remains that you must have a well rounded group of multiple talents to counter any situation that may arise. For example, you enter a Vex dominated raid with the Hive also being present. Now you would need a decent tank to handle the minotaurs and husks on the ground but there are a dozen harpies hovering near the ceiling with some wizards on high rises. You would need a couple of hunters to either help mop up the husks or snipe wizards/harpies from far away. Not to mention at least one radiance lock for healing and buffs. That is just one occasion, not even a boss fight! TL;DR "Will we need one of each class to be effective?" Yes and no depending on what you are planning to tackle. More difficult= more variety to handle "Will there be enough overlap to mix-and-match the classes even in tough situation?" Same as above.