I disagree completely with the reject reason. First, the person who rejected the edit was not a moderator and has no crystal ball. Therefore, it's impossible for that person to really know without a doubt that the post was going to be deleted.
Secondly, deleted content on Stack Exchange sites is not really completely removed from the system. On beta sites, any user with at least 2,000 reputation can see deleted posts. Now, I've seen cases where a question was closed or deleted simply because some blatant grammatical and spelling issues made the question appear to be very low quality, when in fact making an edit to clean things up a bit helped expose the ways the question could be further improved and made to fit the guidelines of the site.
Lastly, as I mentioned previously, making edits to questions that are closed or deleted help eliminate unnecessary distractions that prevent users from seeing them in their true light. However, it also has the opposite effect for questions that really, truly do have no value. Anyone with 2,000 reputation or higher who sees the deleted question will know that it wasn't hastily deleted and that effort was made to improve it.
The Stack Exchange community team has emphasized that a good first step on any closed question is to first edit the spelling and grammar to see the posts more clearly. Is getting a clearer picture of a question that, at face value appears to be a broken window, really not what we want to encourage out of our user base, just because of a couple reputation points?
Post is closed and will be deleted. I don't see a reason to edit it (any edit reputation will be lost, as you know).
With that said, it's clear that this question has no value here, edited or not, but are the high rep users on this site who have the power to reject edits really that worried about someone else gaining +2 reputation for trying to improve the site? If we make this about rep, we discourage healthy behaviors. If someone is trying to improve the site, judge their actions based on that.
It is not enough that I win, everyone else must lose....
-- Genghis Khan