Crossguild and WG are at war with each other. Marines only captured one Warlord. The abolishment of them was a mistake. Doffy knows both shadow leaders. He was an underworld broker before. He will bring Crossguild on Imu's side. Luffy will be supported by the Revolutionaries. Only fair that Imu gets help too. That way the Mihawk vs Zoro fight can happen during the final battle.