Opinions for, by those against their topic, are usually valued higher than those for, by those in favor of their topic. It's more likely to be grounded in reality.
Maybe because 'opposite' is not an accurate statement, when the issue isn't a simple dichotomy.
In fact, one reason people might make these caveats is an attempt to ward off those people who seem to think every issue is an absolute dichotomy, and respond on the basis of that fallacy. I can't say that it is a very effective tactic.
Opinions for, by those against their topic, are usually valued higher than those for, by those in favor of their topic. It's more likely to be grounded in reality.