“In my humble opinion” is one of the most passive-aggressive things anyone can say. There’s this old saying that if one says one has humility, immediately one doesn’t. Obviously, one can simply write “in my opinion.” By injecting the word “humble” into it, one is basically saying, “See, my opinion is humble, so obviously yours isn’t. Also by claiming my opinion is humble, I’ve just hijacked the moral ground from you, so you’re stranded. If you attack my opinion, you’re attacking someone who’s self-consciously humble, beneath your exalted self. If you don’t contradict me, you let my contentions gain legitimacy by default.”
As for me, my opinion is my opinion. There’s nothing humble about it, and I don’t see any reason why there should be.