Complete Warrior said:
Meeting Class Prereqs
It's possible for a character to take levels in a prestige class and later be in a position where the character no longer qualifies to be a member of the class. Alignment changes, levels lost because of character death, or the loss of a magic item that granted an important ability are examples of events that can make a character ineligible to advance farther in a prestige class.
If a character no longer meets the requirements for a prestige class, he or she loses the benefit of any class features or other special abilities granted by the class. The character retains Hid Dice gained from advancing in the class as well as any improvements to base attack bonus and base save bonuses that the class provided.
Straight from RAW.
Now, if a mage who used Sanctum Spell to qualify for his PrC did indeed lose access to his class abilities when he leaves his sanctum, then so would a wizard who cast all of his 3rd level or higher spells needed for Geometer, or a Mindbender who didn't prepare Charm Person on a given day. No. This is not true. The rules don't care if you are or aren't able to satisfy the prereqs at every instant. They only care that you have the potential to fill those prereqs. You have to be ABLE to perform the prereqs, even if you aren't performing them at this exact instant, or are unable to perform them any more times per day.
Imagine a Barbarian with a rage based PrC who "fell" because he used all of his rages per day? Or a Paladin with a PrC requiring smite who already smote his last smite?
Losing an item is different, since without the item, you no longer have the potential to do X. You no longer have the potential to evade fireballs without your Ring of Evasion, no mater what your saves are. If your Str was only 11, you couldn't use your Power Attack feat without Gauntlets of Ogre no matter how much BAB you had.
But spells, and other per/day resources, are much more fluid. They come and go. The rules don't care if you can do it now, just that you could do it. A mage in his sanctum could cast a spell of 1 level higher using Sanctum Spell, so he meets the prereqs. Period.
If you don't like it, and won't allow it, thats fine. I only allow it for entry into a few PrCs. You won't get into something like Initiate of the 7 Veils any earlier, but I would allow a Wiz2/Clr1 access into Mystic Theurge, which needs all the help it can get.