One should either:
A) Be asked for their opinion, which makes it necessary to speak your mind.
B) Have some kind of external goal. You are venturing out into the marketplace of ideas and submitting something for consideration, your objective is to persuade. Besides your opinion, you should have two other things on hand - a suggested course of action which you're lobbying for, and a collection of arguments to support it.
I should tattoo this one on my body somewhere, so I don't forget. I'm catching myself idly publishing my opinion on facebook (even in cases where I don't know what I'm talking about, or don't feel strongly about the topic). I feel like things would be better for myself and others if I didn't do that. Here's a few good reasons why.
- It's vain.
- It hurts my credibility by inviting (righteous) dissent.
- It's rude to others because it adds unhelpful noise to a conversation.
- The "Who gives a shit?" factor. Having an opinion on any particular topic is NOT extraordinary. And treating ordinary things as if they were extraordinary is cynical and just plain wrong.
