I remember listening to an argument on the Today programme one morning a few years ago. A government minister and a woman from a pressure group laid into each other for 10 minutes, culminating in the minister saying, "of course, Mrs X [well actually, he called her Judy] is entitled to her opinion, but ...". At this point I dropped my spoon into my Rice Krispies. He had spent all that time disagreeing with everything she said, and then says she is entitled to her opinion!
I pictured the next day's newspaper headlines: Minister admits that slaughtering the first born in poor nations will solve world overpopulation.
I recalled this while listening to the radio this week. The foreign minister of Pakistan said that the CIA was "entitled to its opinion" when it questioned Pakistan's commitment to unearthing bin Laden. In fact he meant quite the reverse, namely that the CIA is wrong and that Pakistan had played a full and supportive role, including holding the coats of the Navy Seals as they went about their grim business. But that's not what he said. The front page of the next day's Daily Mail reared before me: Top-ranking Pakistani admits his government is either incompetent or dishonest.
What he meant of course, if he meant to mean anything at all, was that the CIA is "entitled to express its opinion", which is quite another matter. I am entitled to express the opinion that Charlton Athletic only failed to reach the Champion's League Final this year because of a world conspiracy between Millwall FC, Mossad and al-Qaeda. But I'm not entitled to hold the opinion, because it's not true. The limits to expressing an opinion are legal and to some extent customary and to do with good taste (although we could argue all night about that); the limits to holding an opinion are to do with whether it's true or not.
Can I recommend Jamie Whyte's excellent book Bad Thoughts, which has much to say about the right to an opinion and how the term is not only used wrongly (as above) but also as a dishonest way of shutting up people who have a contrary opinion, or one that is more valid than theirs, while trying to fool you into thinking they're being tolerant. They aren't.