Since you are likely referring to Rumble, where they let you enter with a 3 person party, let me point out something. This is usually done by players who are not good at pvp. Boosting like this is good for other pvp game type players because the boosters aren't wrecking some team's chances in Clash or Control with their bad pvp play. This is good for the other guys in the rumble match, because they should stand a better chance of getting a win with 3 bad pvp players. In fact, Bungie should set up a special game type just for this so I can finally win some rumble games for my grimoire score! (once I finish my Thorn bounty so I can use it there on them)