There are some people who rarely gives out their opinions. If they ever give out their opinions to others then it just goes to show that the person they are saying those stuff is a very important person for them.
Some people just want to be heard. Some people are not assertive enough to speak their minds but that does not mean that their ideas are not worth hearing. Sometimes it is those people whose ideas have more meaning than those people who always speak up.
So if you know someone who speaks his or her mind rarely and disagrees with you don't fight back at once. It's either hear his or her side first then provide an intellectual argument or try to think and re-think about his opinions.