As I already said, if you matched up "matchmade teams" against "matchmade teams" it would be fine. Would you likely have a better chance at going flawless by playing all games with a pre-made fireteam? Of course. But not everyone needs or cares to go flawless. The point is, people should have the -option- if they want. And as I said in another comment, if Bungie feels iffy about that, they could easily do a lobby system where you open or enter groups.