"But why would you want *easier* access to some prestige classes, they are supposed to be difficult to qualify for and they are supposed to provide power at a cost. I really don't like this idea at all. "
I'm not talking about easier access, I'm saying if your Rogue/Wizard becomes an assassin, he shouldn't be starting over again casting magic. He already knows more about magic than a typical assassin. I would keep spell list seperate, I've taken the spells to psionic powers convertions from "If Thoughts Could Kill" and limited most of them to prestige classes, a Psion doesn't manifest Fireballs, but a Pyrokineticist can.
"For some Prestige Classes, this will not work well. Bladesinger, for instance, would be much nicer if it added to spellcaster level, but...one of his abilities is to cast a Bladesinger spell as a free action every round. To give this ability to a high level wizard would be very unbalanced. "
Forgot about the Bladesinger and the Sorcerer can become unbalanced very easily.
"There is one niggling issue with the Sorceror class. Advancing in spellcaster level does not gain them additional spells known. (In the case of the Wizard, you would not get to write 2 new spells into your spellbook.) "
+1 level gives Psions new powers discovered, additional power points, additional 0 level manifestations and improved psi-crystal abilities, so it should give Sorcerers new spells known and cast/day and improved familiar as well.