Spoilers are used on forums that have large tutorials, or discussions. I see spoilers used daily on HF, and my forum (just installed plugin sadly to use it).
TO break up big tutorials, as said
Not all forums have large tutorials or lengthy discussions. Most don't, in fact. Sure, HF fits that criteria, and yours might too, but no other forums I've ever been a part of have lengthy posts that necessitate this being put in the core.
Why you'd have a plugin installed for it, I don't know, seeing as it just needs to be a MyCode and quick JS edit for SCEditor integration.
MyCode spoilers can't be nested easily in 1.x IIRC, that's why plugins are used.
I see nothing against including spoiler in case of high interest (a matter of few lines of code), but also nothing that describes it as a "most common must-have" feature - personally, I had it only on one of my forums and it was used very rarely. Especially when you complain about installing one additional plugin which requires few minutes of work and adds nearly no additional overhead..