I think so. Even in cases that the letters don't get sent to the person in question it can help us to work through thoughts and feelings ourselves. In cases where it is sent to the other person, its a great way to be able to edit out thoughts, and be more rational about what you are trying to say. Emotions are a thing of the moment and when we try to talk to someone live and face to face, since emotions are of the moment, They can get in the way of being objective and rational in our conversation. While emailing or writing a letter we can slowly work through the emotional moments, get it out and if we find that it's too emotionally charged, edit it out after the fact. This cannot happen in real time conversation as what we say in emotional moments remains forever "out there".

So if it's something that is delicate and sensitive, yes, I think writing a letter could be a great way to deal with it.