On one hand I am happy that we have things like Marcus Aurelius' Meditations, on the other I feel that peoples private and unpublished writings should be kept private. It's reasonable enough to let a limited number of researchers access private documents, but the broader public, no. This isn't a letter meant for us, it's not meant for anyone to read and I feel that publishing it is a little disrespectful.
I found it very beneficial to read recently so I'm grateful that his family chose to publish.
I enjoy "Letters of Note" and many of the most impactful letters are personal ones between private individuals.