Interestingly as nice as the ability is for various Pathfinder classes I like it most with old 3.5 classes in the Dread Necro, Warmage, Beguiler vein.
Depending on the leeway a DM gives it could be used to get early access to some spells which I'm sure some think warrants book throwing as well. Oh no, the Sorcerer has Haste from Summoner list as a 2nd level spell!!! Oh no the Sorcerer has Cure Light Wounds from the Bard list!!!