Im not saying you have to play as a team but there are plenty of people who think they are good because they only play control/clash with a full party of equally skilled players.
It matters a lot. Especially when heavy ammo drops.
There's always the kid who gets the heavy all to himself.
5-6 heavy ammo users on the fireteam vs 1-2 selfish randoms on the other team = Killstreaks will happen.
Playing with a team of 6 in control just means you can control the spawns and just train supers for the whole game.
Ever played a team of 6 gunslingers that are partied up? If not you better hope you dont