Well you know if everybody stated,"in my very biased and totally unsupported opinion...............", there shouldn't be any need to ask that they defend their statement. Unless it is interesting and I want to know how they arrived at that commonly held but misguided opinion.
But when they state an opinion as fact, and I believe that "fact" to be untrue, I just have to ask where they got the information. If it is legit, then I will have to change how I see the issue. If they are just mentally just blowing smoke, I have to challenge it.
Call it my limited effort in supporting Truth, Justice and the American Way.