User blog comment:FairyRaven/Fairy Tail Chapter 386: Galaxia Blade, Review and Commentary/@comment-24066070-20140531100609/@comment-7418318-20140531102038

Good question. My money's on the latter.

Maybe summoning them from the book is harder, more costly, more time consuming or less practical by some other means than reviving them in the tank and that's why they had Hell's Core in the first place.

An interesting twist would be if Demons could only be summoned from the books by Magic (and not with Curses) and thus they have to rely on Hell's Core to operate unsupported by Humans or other non-Demonic species.

And so far they've been so convinced that nobody could ever reach and destroy Hell's Core that they didn't think it necessary to have a sufficiently strong non-Demon along just in case they need re-summoning.