I was just going to say this but I got beaten to the punch. Its the laziest thing that you can say: its basically saying that you know that your belief is wrong or based on absolutely nothing but you don't want to defend it or change it. Its especially annoying when someone says something that's factually wrong, you say "well that's a load of rubbish" and quickly explain the facts only to get "durr my opinion!!"; just shows a lack of self-consideration that I really don't like. Then again I've got a Politics degree; so I'm not exactly normal...
Its also wrong, the sport that most resembles an organised fight is Aussie Rules