It's often very difficult to determine another person's actual motives are. In this particular case, there's no way to know whether someone takes a side because of their bias, or has a bias because their honest opinion led them to invest a personal stake in it.

If the arguments are actually good, it doesn't matter what motivated a person to make them. Casting aspersions on motive is just a cheap way people avoid dealing with good arguments they don't like, perhaps because of subtle biases of their own.