The basic rationale for allowing post-review to have an "email only" mode which only sends out notifcations without really posting a diff on review-board is the following:
First, it allows one to easily write VCS commit/push hooks which choose whether to "post-patches and notify" or "only notify" based on which user commits something. Useful when the patches of certain users are required always to be reviewed, and the patches of others by default do not require review. Furthermore, toggling whether a review is or isn't submitted could be done with certain commit-message flags. Granted, only sending emails could be done in a commit hook without needing to use post-review, but using it would greatly simplify things, and prevent the need to duplicate email address lists, etc.
Second, it would work very nicely with a revamped-reviewboard (of the future) which allows one to review the code as they please with their standard diff-tools, and if something worth commenting on stands out, to log into RB and generate a review-diff on-demand by specifying the revision/range-of-revisions it should be over, and then creating the review like usual in RB, adding comments/etc.
Presently using RB 1.0.9
