Conventional wisdom is that more spell levels are better than more spell slots of lower level. For example, this argument has been used to argue in favor of playing a Focused Specialist Wizard. From this principle, it is often inferred that even an early-entry Mystic Theurge is inferior to its single-classed parent classes. It is claimed that even though having both arcane and divine spellcasting on one character ought to be more versatile than only having one of those (but at a higher level), it almost always isn’t because the extra spell levels really do add that much versatility.
Taking the above as true, my question is this: Does there exist any Mystic Theurge build that claims to gain more versatility from mixing two spellcasting progressions than it would from only sticking to one progression? As an example, I’m pretty sure that divine and arcane necromancy can be mixed to give results that could not be gained from just one progression.
Rainbow Savant builds can be ignored. Answers do not need to prove that the build in question actually does what it claims to do – they only need to say why it claims that.