Why should you deny yourself the power of Arcane Spellcasting, Divine Theurgy and Psionic Manifesting all in one build?

Human StP Spell-Focused Erudite/9 Ur-Priest/2 Psychic Theurge/9 and BAM! The universe is now your whipping boy... However that can be achieved just as easily with any T1 class... This is just a T1 build that gets access to 3 spell list and is able to do just about anything if it really wants to