First of all, sure, of course it is encouraged to fix errors in other's posts, so yeah, if someone edits a post, it's great if he fixes any errors he notices. However, this is not required at all. Every edit that improves a post is, well, an improvement.
More often than not people just don't notice every possible error in a post or don't have the time to fix everything throroughly, or just have other priorities to fix first. I for example1 am usually not going to read and throughly analyse every possible question or answer I see, either because it just doesn't interest me or I don't know the movie the answer is about at all. However, things like incorrect tagging, a "thanks in advance" or incorrect spacing around brackets or punctuation stick out like a sore thumb and are pretty easy and urgent to fix. Those are things that definitely need to be fixed, they are in absolutely no sense "sloppy/pointless" edits. To answer your question, the benefit of such an edit is that it improves the post and that is a good thing. And you know what, I'm very thankful for people who do at least that, even if they don't fix the whole post into utter perfection. Afterall editing and improving posts on this site is an iterative and collaborative process, it's not only about the original poster creating it and the first guy coming round the corner perfectly fitting it into shape.
So other than this meta question, I don't think there is any way, let alone proper reason, to demand editors to either be perfectly throrough or else refrain from it, just because there might be people who think an edited post is automagically perfect and refrain from any further consideration of it.
(Disclaimer just in case: Of course this discussion is entirely independent from the problem of people editing thousands of year old posts at once for nothing but missing linebreaks, which I would rather not encourage.)
1) Note however, that this answer is not about me or that edit alone, but a general answer. In fact I didn't even notice that the question's example seems to be about one of my own edits before considering and answering this question.