If I remember correctly, it also comes with some penalties to saves against cold, which is not all that significant, but it's there. My guess would be that the item was added to help non-casters cope with some of the many enemies that make use of will check spells, as well as the many areas that give a significant penalty to will saves, like the crypts in the Vallaki Western Outskirts. Since Mind Blank spells protect against many of the will check spells, the bonus on the item doesn't effect mid-late game content much as people are often expected to have a mage who can provide it, and it offers options to manage without one, or help with the earlier portions of the module.