[3.5] What do you think of Mystic Theurge?

[Optimystik]

I do have a question for the other readers, though: when you take a class that lets you advance in spells from a previous class, is there any particular reason why such classes can't be stackable? I've been looking around for a reason for why that shouldn't work, and I've been inviting my DM to look around for a reason for why it shouldn' work (sadly, we both have a habit of missing key sentences in the middle of rule texts so that we get big pictures but not finer details).

As DW said, PrCs like Mystic Theurge and Archmage do not have a spellcasting progression - they only continue that of a base class.

A PrC with a casting progression is one that gets spells on its own - like Sublime Chord or Ur-Priest.