That is correct. Categories have permissions, notifications and similar features. Discourse-Tagging is mostly to allow for cross-category taxonomy. At the moment, you can not filter on that taxonmy. Ergo, you would always have to subscribe to a higher-level category. However, there is – per default – a weekly digest of the things going on in the entire forum (minus categories, you muted). So I think it should be quite easy to stay up to date on things you care about.
In general I am (and DC is designed that way, too) in favor of less categories. Only create a category after there has been significant content for it. I'd vote against a GA2015-Category for the simple fact, that it only has three topics. As topics are good in encapsulating one conversation, what is the point of the category? Well, "finding it in the archive later", you could say. In that case, that'd be taxonomy and if the search really isn't enough, then we should install discourse-tagging, too.
I am rambling and repeating myself. You get the gist .