By the reasoning of the OP, then nobody has any excuse to not exclusively play single-classed full casters -- but the game is so much more than that.
As food for thought, try to look at this from a systematic perspective:
Let's compare a theurgic caster to the CR and LA systems (I know that they are imperfect systems, but it's what I've got to work with) --
- a 10th-level theurgic caster has the casting of 2 7th-level casters
- Two CR 7 creatures is a CR 9 encounter
This would seem to indicate that a 10th-level theurgic caster would only be a CR 9; but one also needs to consider that said caster also has 10 full HD (and, thus, all that comes with that -- HP, saves, BAB, skills, etc.). Sure, you may be limited by action economy -- but with prep time and/or proper contingency planning, this is a minor obstacle.
The whole "X+X = X+2" is also demonstrated in gestalt characters -- (by my observation) gestalt characters are generally considered to be about a +2 LA -- this would mean that a 7th level gestalt would be an ECL 9. Surely the theurgic caster's extra 3 HD can account for something, right?
Or maybe we create a new race; and let's say that race is a 3HD creature with a +0 LA. And let's say that race's only ability is that for every HD it has (racial or class-level) it also gains spell casting as if it had also gained a level in class "X" (and just to avoid silly shenanigans, let's say that whatever "X" is, then he cannot also actually take levels in that actual class ; and let's also say that he does not advance this ability when he takes levels in a MT-esque PrC).
Would such a race make for an attractive character? Would he be preferred to being a single-classes caster with a standard PHB race?
Or, lets put that ability on a 1 HD race -- surely such an ability would be worth at least a +3 LA. Again, would such a race make for an attractive character? Would he be preferred to being a single-classes caster with a standard PHB race?
Again, just some food for thought.