"In my opinion" (IMO) is often not necessary in a post, but sometimes you like to separate your thought(s) from facts or things already said by a previous poster, although that is next to impossible sometimes (originality). I often edit a post and add or subtract "IMO", because as you both point out, much of what is posted is our opinions, so to state it could be considered redundant. I will go on record as saying that most of what I post is my opinion, but often based on things I read. If there is a direct source or fact, I try to include that.
Lots of grey area as well, for instance Captain's statement:
If you remove his "IMO" another poster is going to try and take what he said as him trying to state a fact and debate him on it being fact or an opinion. In this case, he made it obvious, its strictly his opinion.