It being private means that it's only visible to admins or Mods as well could see the note including the user himself or nobody else except the user can see it?
I see the reasoning behind this suggestion; I would support this as well. Perhaps I had the impression that someone was farming trust, but had no real basis for the suspicion. Then I could simply write a note (visible only to me) on his profile for future reference. Same goes for if I thought someone was trustworthy, but I didn't want to put a positive trust rating because it was only a personal opinion and I didn't have any direct dealings with the person. It could even go to other small notes that I would want to write down for future reference, but wouldn't want other people to see/affect the person's trust rating.
This reason sounds good but for users who have left a lot of ratings, they would find it tough to scroll through those ratings to find this private rating. It would need users to have another tab of Private ratings as well.