There are a number of “theurge” Prcs in D&D 3.5 and D&D 3 — classes that progress both their own and the base class's spellcasting — and I've notice a lot of hate for them that I don't understand. Why all the hate?
I've found many discussions online where people ask for advice for builds with such classes, with answers that are mostly negative.
Some are just outright dismissive. They range from "this class is utter garbage" through "only for fun and acting" to "only useful with cheese strategies revolving around early entry combinations or caster level enhancement tricks".
Most such replies are only semi-correct in very narrow range of character levels (let's say 5 to 15). They're completely, utterly wrong for levels 15 to 40+ (also known as epic/mythic), since progressing beyond level 20 nullifies most of the issues people raise (like having only 10 levels of double progression class, since epic levels allow taking levels beyond a class's standard maximum).
Other common popular replies revolve around basic attributes required for casting, but don't seem to realize (or simply ignore) that Wizard/Archivist isn't the only combination.
In fact, there's nearly infinite combinations using same attribute, considering all the base classes. Each type of caster (arcane, divine, psionic, etc.) comes in several flavours, witch (arcane, wisdom), mystic (divine, charisma), wilder (psionic, charisma) to name just a few.
People also argue against them based on the action economy.
This one is also easily solved using cloning, schismas, time-stop-like effects, casting through crystals or familiars and several other methods of circumventing limits on actions taken. Which is also better: the more such effects are available to the player with every casting class having their own.
Besides, once the character gains level 9 spells (or equivalent maximum) in both base classes, and reaches character level 21+ for epic spells there is no downside compared to single-progression caster. Versatility from having access to two full spell lists also translates to epic spell creation (which is based on skill points and "spell effects" known instead of actual strength of spells in 1-9 spell lists).
So I wonder if I'm missing something:
Are there any downsides for level 21+ double progression caster characters?
Is there any mechanical benefit of playing single-class caster character I'm not seeing? (I understand the storytelling and acting arguments but let's ignore those in this topic.)