So there's this lovely little piece of equipment that a theurge of mine would very much enjoy having, being as his 'thing' is having a scroll on hand for every situation. It gets expensive, as you can imagine.
What is the reasoning for restricting this to spontaneous casters?
What would the ramifications be for allowing prepared casters to use it?
If you would let prepared casters use it, would you change it? How?
Maybe it's because scrolls are so much friendlier to prepared casters than spontaneous ones? Wizards get Scribe Scroll off the bat (letting them scroll up niche spells at will), and there are so many nice spells that a Cleric can choose to scroll whenever they have free time.
Sorcerers/Oracles? Stuck to whatever spells they chose (even for scribing scrolls themselves) or whatever scrolls they bought at full price/found. This item seems like it's supposed to help spontaneous casters utilize scrolls more efficiently.
Just my thoughts.
It doesn't work for prepared casters because it is an item that essentially replicates a feature of prepared casters. It basically lets a spontaneous caster learn a scroll spell just like a prepared caster would, but doesn't consume the scroll. Instead they lose access to the spell as soon as the scroll is consumed.
Prepared casters get a whole bunch of nice things going their way, like being able to use metmagic rods on the same round they pull them out of a Handy Haversack and getting Pearls of Power at half the cost. Plus getting stuff a full spell level earlier. Mnemonic Vestments is one of those few things that is better for spontaneous casters. Also, wizards get access to a bonded item, which more or less lets them do the same thing.