User blog comment:12th Supernova/Panic in the Streets of Dressrosa/@comment-10543621-20140424000905/@comment-24.231.163.200-20140424013611

Even if Doffy took control of someone they are only as strong as he can make them punch or swing a weapon. He would not be able to use Luffy to his true extent, such as any of the gears or haki. So unless he took control of someone who was solid to use as a weapon or something it is better for him to just fight on his own accord.