Does nothing to people who never check the forum (i.e. your average commenting person) as it's not present in the commenting version of fast reply.
Precisely my point. There is no option in fast reply except entering bbcode.
In the Special BBCode drop-down (3rd button from top left corner), spoilers are 3rd and 4th from the bottom of the list. Doesn't mean it couldn't be more prominent, but I think this part of the site coding is directly inherited from the parent BBS system. It's not customized for Batoto at all.
If added to the fast reply, I would prefer it to be just as prominent or more so than image/media buttons, since those require spoilers, meaning spoilers should be used no-less often then them.
However, as you said, this is likely in the domain of the BBS subsystem. Adding/removing elements may not be as easy as it sounds. I would think that the BBS system would have configuration options to customize this toolbar "easily", however I know nothing of the system in use, and reading the documentation doesn't sound like anyone's favorite pastime.