This is an excellent point if the user is completely new to the matters discussed on the forum, so it will apply to casual surfers. However, such users will likely land from a search result and perhaps reply or post a new topic in the same category, so categorization matters less.
More commonly, users are familiar with at least several categories in the forum and may prefer a category-centered view. I’ve expanded upon that in:
My feelings on the landing page aren’t very strong though.
My biggest problem with categories is that they don’t map real epistemologies very well. Information/Revolution by Prof. Michael Wesch of The Machine is Us fame describes the problem very well:
Say you want to discuss the correlation between sleep and weight loss on a health forum. Instead of posting in one of the categories and perhaps cross-posting in the other, why not tag the topic with both tags? By the way, the mere existence of cross-posting is a sign of the limitations that categories artificially impose.
Practical question: how would a Discourse forum with only tags and no categories work? Is there one out there?