>>724208996
summoning tends to fuck up boss AI, it will randomly zoom back and forth and let you get away with so many more mistakes and panic rolling than if it was focused entirely on you
It also at minimum doubles your potential damage output, usually significantly more since there's a lot more damage windows while it focuses on the other guy
And it introduces the potential of carrying. Yeah sometimes you'll get a useless fuck who dies in 30 seconds, but sometimes you get someone that can just solo the boss while you only have to not die. If you do a lot of invading you will see a lot of these players, at the endgame fatrolling with 1200 HP and a greatshield that clearly got hard carried through literally every bossfight by summons. It might take a few tries, but if you get lucky with the phantom, it is literally an easy free win, someone else is straight up beating the boss for you, barely better than just handing the controller to someone else.