To be perfectly honest, in the end it really comes down to the game and the player. In a game where the DM bans Leadership and their's not enough party members to fill every role needed, then theurgy may look very tempting to the party's spellcaster. A campaign where there's a large number of players and the use of a cohort would just overcomplicate things or detract from the enjoyment of other player, then once again, theurgy might look like a very tanilizing option to someone who wants some more versatility. A gestalt campain where the DM actually allows dual-progressioon prCs, theurgy suddenly becomes the Holy Grail to a gestalt dual-caster.
I'll admit, I didn't intend to go theurgy with my character in the current campaign. But we kept losing our clerics... RL events kept pulling them away from the game and we've been unsuccessful at working out a new schedule that works for everyone... as a result we needed someone who could fill the role of a cleric atleast short term. I saw some spells on various divine spellcaster lists that would benefit my character, and had recently discovered the Archivist class which opened up the entire divine spell list to me, and fit in quite nicely with an RP element of my character (she's obsessed with knowledge). So I picked up a couple levels of Archivist over a couple months while we waited for one of our cleric players to be able to return to the game... (still waiting on that...) Since I invested a few levels into it already I decided to just stick with it and go theurgy. Had we known that our lack of a cleric was going to be long-term rather than the short-term we had originally believed it was going to be then one of us would have most likely picked up leadership and taken a cleric cohort instead. But Honestly, I don't regret picking up Archivist and buillding into theurgy one bit. Sure I'm not as useful as having a Wizard & a Cleric, but who really cares. I provide the functionality that our party needs of me, and my role as a wizard is an Illusionist anyway, I'm not a blaster, and I was already behind on spells from the start since I started out as a Spellthief. Took up Illusionist at 3rd level to get a better selection of spells and higher level spells in the end. Even with Theurgy, I've come out ahead of where I started out. Originally I was only going to have a handful of spells capping at 4th level... now I've got tomes filled with spells up to 8th level, and when we level next i'll have 9th level spells.
Also, epic level campains are another spot where Theurgy is a good option. And honestly, it's not about the number of spells you can cast per day. But about the versatility of said spells. As a prepared spell caster being able to prepare more spells each day even if you will only ever cast a handful of them allows you to be better prepared for the encounters to come. If you know you'll encounter a specific creature or obsticle, then you can always prepare spells to deal with it and judge how many of what you need for it. But it's what you don't know about that causes trouble. A theurge can deal with the unknown much easier than a straight caster. Simply because they can prepare for more possibilities at once.
