For me, the main advantage of "bzr pull --preview" would be to be informed about the merge conflicts that had to be expected. In that case, "bzr merge --preview" doesn't work since in most of the time it would fail due to "uncommitted local changes".
So, for me, the ideal output of "pull --preview" would be what "bzr st" would display after actually having pulled.
For me, the main advantage of "bzr pull --preview" would be to be informed about the merge conflicts that had to be expected. In that case, "bzr merge --preview" doesn't work since in most of the time it would fail due to "uncommitted local changes".
So, for me, the ideal output of "pull --preview" would be what "bzr st" would display after actually having pulled.