Well you don't NEED to supply Matchmaking though you certainly need one or the other. But you're missing out of large percentage numbers of your player base if you choose not to provide it or alternative routes for those without premade teams. Players who feel they're cut out of the loop won't keep with the game, won't spend or support the game. Whether that's a smart or bad business move depends on whether the privileged players spend more than solo and premade teams combined. They may - no one has those statistics except perhaps Bungie.