I'm most guilty of using the phrase when asked by someone what I think, and I have an inkling that they may not like it. It's not that I am normally dishonest at all, but I may drop the diplomatic approach I strive for and be more blunt.
I also use "in my opinion", but I don't say "just my opinion". I know it's rather obvious it is MY opinion, but I'd rather clarify that than have someone mistakenly think I am arrogantly stating my opinion as if it is fact.
Ah, the effort an NF puts into delicate phrasing .