Has saying I told you so ever done any real good? Ever made you or anybody else feel better? Ever earned you the title of a-hole?
"I told you so: is always a deliberate dig at the other person. It contributes nothing to the productive conclusion to a situation. While the person who says it is trying to demean the other, the or she is demeaning himself/herself as well.
I use it as a learning tool. Sometimes it can get someone to listen to the first time you tell them somthing. I've told some of the guys I've trained that a lot to intentionally piss them off. It gives me a little pleasure but more important it makes them remember. 99 % of the time they don't make the same mistakes twice. Get called an a-hole every single time.