Since the topic is very vague, it's hard to comment on this, but I'll go over what comes to mind.
An opinion is a great way to pick a fight. Will I walk up to someone in the street and tell them their hat looks silly? Nah, and I don't know many who WOULD. Not afraid to, but I'm smart enough to know when or when not to. Fear is the result not knowing. Lucky for me I know what can and can't happen. Believe it or not, people WILL kill/hurt others (read the news lately?) for the tiniest of reasons these days.
In most situations, a personal opinion does not really help. Here's a quick example that comes to mind. Recently a new co worker joined my team. My superior called him "fat" (he didn't even look fat in my opinion). Was the opinion useful? No, and it created an offensive atmosphere. This same superior always makes cracks about a guy who is bald and calls him "weird looking". Those two are practically always verbally combating each other now. It's even worse when a group of people start giving their opinions about someone and start calling them "stupid". But that's an entirely different subject so I'll stop there. >_<
If an opinion is going to hurt someone and help in no way at all, I don't see a point in unleashing it. I've witnessed the results of destructive opinions enough to know what they are capable of doing. Unless a person wants to change or improve themselves, opinions are nothing but wasted breath/keystrokes (in my opinion xD).. If an opinion helps me earn more money, benefits me, a loved one, or others that need help in some way, then I'm usually more than happy to offer my opinion though. :)
As for opinions on games, music, and other forms of entertainment? I don't mind sharing them. :)
Once again the topic is vague so it's hard to give a solid response. The situations of when or when not an opinion is given can easily change depending on who (stranger, friend, co worker, acquaintance, classmate, relative, etc.) is giving the opinion and who is receiving it.