[Retra]

You can easily hide bias behind good arguments. Attention to motive is how you discover this bias. Humans are notoriously awful at dealing with bias, and you're suggesting that people remove one of the only tools they have against it?

[DennisP]

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.