As detailed in this question, the API doesn't seem to properly return the body for comments. This is a fairly big issue for me, because it means that whenever I want to get a question and its comments, or an answer and its comments (read: "Quite often"), I have to use two API requests, one to get the question/answer and one to get the comments. This 1) gets messy in code, 2) results in more network requests and 3) burns through my API rate limit at double the speed that it should.
body
field in the filter does get me what I need, as mentioned in one of the posts you linked to. Still a little bit of extra data for mobile users, but at least it's a lot cleaner than having to load comments separately.