I can see what you mean. You can always place a message on the postcards page stating to visitors that their postcards may be viewed by the administrator.
I wrote this mod more for security of my visitors. With about 97% of my visitors being under the age of 17, it's important that I keep my eyes open for anything that may seem odd. When there's children involved it's my job as an administrator to make sure they are kept safe on my site and that they don't see anything that is unfit.
But naturally if this wasn't the case and I ran a site which involved more mature visitors I personally wouldn't bother logging messages, because it's a full time job and I'm basically not really interested in what people have to say in their private conversations.