You act as if a sorcerer would get dozens of these items. You overlook the fact that sorcerers will usually learn the more useful spells to begin with, meaning only lesser utility spells would be gained. Add to that the fact that you can still only cast so many, and I don't see any breakage at all. The limit to spells known is not their fundamental limitation or penalty, it's just part of the class. Allowing such an item is no more broken than a helm of teleportation (which allows even non-spellcasters to cast a very powerful 5th-level spell) or, say, a holy avenger.