I was flickering through the Av Forum and came across the line "It's only my opinion" several times and more than often this ... uh .. statement was used as an exuse for not providing facts or at least an idea that explains the authors point of view. It often goes like this:
User A: "Competitor A does not have a business plan."
User B: "Please elaborate ..."
User A: "Hey, it's only my opinion!"
Eventually User A stresses that it's his/her right to express his/her "opinion" and that the mods would do a good job in supressing it.
While opinions differ from facts (such as 1+1=2), because opinions are disputable, they do NOT exempt you from finding arguments that back your opinion up.
According to Webster and Encyclopaedia Britannica, an opinion is "a view, judgment, or appraisal formed in the mind about a particular matter" or a "belief stronger than impression and less strong than positive knowledge" or "a formal expression of judgment or advice by an expert."
"I like the 777 better than the A340, because it's roomier" is a valid opinion: You have compared the comfort both aircraft provide and came to the conclusion/judgment that you like the 777 better.
"Competitor A does not have a business plan" is not an opinion, it's a wild guess, since you can't back your judgment up with more than maybe a vague impression.
(Besides: you don't even do Competitor B a favor by saying so. If B looses quite an amount of marked share to a competitor that doesn't even have a business plan, either A delivers a truly superior product - sheer geniuses at work - or B is run by a bunch of morons).
That being said, it's not wise, rhetorically speaking, to say "it's only my opinion". You are merely stating the obvious because everyone knows you are not somebody's spokesperson, and it's simply unnecessary blablah to state the obvious.
But foremost, and thanks to the little word "only", it sounds like a defense and instead of defending yourself, you usually want to lead a discussion.