I am continually vexed by this limitation. I have folks on my team for whom I wish to initiate code reviews on a case by case basis. I do not wish that they create an enormous number of reviews that I don't have time to handle, but rather that I peruse the check-ins and comment when needed. I wish to make comments on their code, have them respond/fix and then close the review when I am satisfied. I don't want them (the author) to summarize (frankly the whole summary thing seems useless to me). I am the person reviewing, and I want to be in control of when it's marked complete.
Currently, the system actively disallows me from setting myself as a reviewer without ceding control of when the review is marked complete.