Have a look at this simple example: https://stackoverflow.com/posts/5349764/revisions. There is a approved suggested edit, and an improvement was added on by a 10k user. The interesting thing is that the original edit should obviously be rejected - fixing one typo by introducing another is... not right.
However, its a small mistake, so the 10k user in question simply went for the path of least resistance, and clicked on the 'Improve' button. The problem here is that the suggested edit is implicitly accepted when you 'improve' a post, netting the editor 2 rep. Now the rep gain may be a token sum, but it does send the wrong message doesn't it?
Would it be useful to have a 'Reject and Improve' button for these sort of scenarios? Or would it be too rare to be useful?