The consensus should therefore simply be such: that where side stories (or even sequels, prequels and spinoffs) exist, they should be licensed, for the Schrödinger complement,
'Is it really worth reading ( ´ Д `)ノ'
juxtaposed against,
'You absolutely have to experience this!! (∩`-´) !"
is that way easily resolved and made a non-issue.
To embark on exploration, indulge or no - that later can be left to the discretion of readers (who anyway will probably, you know, read, 'cos they're fond of the world)
ReZero was brought up earlier, a series with 20 volumes worth of apocryphal content, effectualized by the author to explore backstories, and even to shape up future arcs, sweetening the overall experience by a factor... pretty high. Incalculable. It's invaluable. Tanpenshuu is great. Yen Press couldn't ignore them forever.
In fact, as of the arc they're translating, and those moving forward, they're pretty barebones ... unless you're incorporating side story knowledge since, well, that's around the time Tappei (author) started fully integrating everything.
(I'm not sure they'll grab the ReZero encyclopedias sadly. Hopefully.)
Likewise for Danmachi, where over half (55%+) of all world exploration and loose ends transpires in SS material; from the unsparing Oratoria, to Familia Chronicle, then up to Tales of Heroes, still further to Minor Myth and Legend, to at last Orario Stories. Omori will probably pen even more.
Ryu's intervention in Freya's arc is truly great once you go through Familia Chronicle, and entry A-R.
(No news so far on Argonaut Record, and Orario Stories on the Yen Press side)
You find the same with SAO (SAOP), Full Metal Panic(FMPA! &FMPF), NGNL, Strike The Blood (No Appends from Yen Press), Date Alive (No Date Alive Encore ) Classroom Of The Elite (This one is so critical, I started purchasing JP copies of the books which appended them from all stores. I also have a book which collected all SS until the end of the second year, and it's a book 1000 pages long. All of that is missing from SevenSeas. Was really hard to find, including V0.), Mushoku Tensei, Princess Tutor, I.D, The Irregular at Magic High, My Youth Romantic Comedy... just about everything that has ever had these, really, and I guess they all do.. Poke around and see what's missing.
For older series, it's a shame, since they've probably lost licenses, but, yeah. Who knows.
I'm looking forward to Strike the Blood Append 1 - 4 lol. Kinugasa's 1st novel too someday, when it's revealed to share the same world as Youjitsu.
Black Iron Magician...
Classroom of the Elite (again) Official Guide Books...
(´• ω •`)
Well, that's it from me.
Side Stories are great.