Subreview papers, to what extent?
(a) Benefits: You mentioned the biggest one, i.e. knowledge, exposure and an updated idea of trends in the field. Besides this, the major factor is generating/maintaining goodwill with the person who sent it to you for subreview. This is an intangible benefit which could multiply in ways that are hard to predict.

(b) When is it not worth it: Probably once you have defended your PhD. You may prefer to review directly. Definitely once you're in a faculty position, unless you do it as a favour to someone.