Empirate
First Post
If this becomes a doube post, it is because it seems the internetz ate my first reply.
There was an NPC Sorcerer/Favored Soul (3.5 oracle-look-alike)/Mystic theurge and she became the cohort of one of the players in my campaign. I leveled her up to lvl 18. Her role was buffer and general support, including some healing. At levels past 12, she shone quite brightly. The party had no cleric or wizard, and she stepped in to hand out many, many buffs, acting as a power-multiplier for the whole party. Rarely did she contribute much directly, so it was a very NPC-ish role.
Important part bolded. Alongside a real caster, she would have sucked goat's balls in comparison. In a party without one, even a theurge build can feel powerful.