Chris Dillow argues against opinion as "just prejudice". This is why raw 'public opinion' is not the proper foundation for democracy. It has no normative significance; the mere fact that people believe something tells us nothing about what is true or ought to be done.
Similarly, a liberal education is not just about enabling students to express themselves. (As Chris says, "Selves aren't interesting." I'd disagree, but it's true enough in these contexts, at least.) Rather, what's interesting is intellectual inquiry. Students should be encouraged to draw their own conclusions, not because any belief will be magically "validated" by the mere fact of their holding it, but because thinking for yourself is the way to develop your rational discernment. The take-home item of value here is not the 'opinion' one forms, so much as the reasons that led to it -- and, even more, the discernment that enables one to appreciate these.
If there is to be a positive case for democracy (as opposed to simply showing it to be less bad than every alternative), it cannot rest simply on 'one man, one vote'. To have all opinions count equally in the tally is a feeble sort of equality, given the general worthlessness of opinions anyway. But to be granted an equal opportunity to participate in democratic inquiry -- to seek, offer and assess reasons in deliberation about the common good -- now that's something worth getting excited about!
At least, that's my opinion.