I've wanted to do this before. That said, I've wanted to do it maybe 3 times ever. Of those 3 times, I probably went ahead and posted my thoughts anyway in team chat twice. (I think it had to do with something that I noticed on the map, but not everyone needed to know about it right then). I don't know about others, but I would RARELY use this. I don't think many other people would either. Plus it's just another check box to be confused by when you're trying to get your message out to the right people. Actually, it would be a radio button, but still. More times than I've wanted to post a note to myself, I've posted something publicly that I only wanted to go to my team. I think that this would only increase with another option.
I would hate it if I said something crucial to my teammate only to find out that I only posted it to myself.
Therefore, I'd say this suggestion is not worth it.