Does bias always decrease the validity of an argument? Can it ever increase its validity?

A biased opinion is often discounted. “Of course he would think teachers are underpaid; he’s a teacher himself.” But does bias always make an opinion less valid?

One might think, for example, that the chronically ill are too biased to offer a fair opinion in healthcare debates. I might argue, though, that they are exactly the right people to discuss the issue. Those who have never been seriously ill may not understand just how important the issue is to some people.

Does being part of a group affected by decisions make you negatively biased or might it even make you more qualified to offer your opinion on a subject? Another example; issues of women’s rights: should women’s opinions in this arena be held more or less highly than men’s? At what point does personal experience cross over into harmful bias?