[3.5] Ways to learn spells from a different class list

[Psyren]

This argument becomes much less compelling when you acknowledge that the similar Expanded Knowledge feat raises the same unanswerable question, but still works on other classes' power lists.

Because Expanded Knowledge explicitly allows this. Extra Spell does not.

Psionics are balanced around potentially any manifester (regardless of theme) being able to access any effect within their power level range. A psychic rogue can potentially summon, a kineticist can potentially heal, a psychic warrior can potentially shapeshift and an ardent can potentially blast. Magic was designed, if not to prevent the above outright, to at least require a few more hoops.