I always feel silly adding it in, because of course what I'm saying is my opinion. What else would it be?
But I say it when I'm talking to people who need it.
I've always thought that the main reason for it is that most people are very easily threatened in their opinions, so without the "I Think", they can feel like they are being attacked.
But it could be that most people simply haven't been trained to differentiate fact and opinion on their own, so they need clear markers.
At least, I think that's possible.