The premium membership is worth it just to get the DRM-free Premium Editions for $6 each that often include the bonus short stories that only were available at certain stores in Japan. Realist Hero in particular has a great collection of bonus stories that really add to it.
List of bonus content: https://forums.j-novel.club/topic/305/premium-e-book-bonus-contents-list/244
(Note: the versions you read online with a membership do not include this bonus content, you have to buy the premium editions of the books to read it.)