Derrick Reeves said:
I'm not particularly phased by his presence, as the party Cleric. He can't fight in melee, can't cast Heal, can't cast Heroes' Feast, can't cast Greater Scrying, etc, etc. He can't turn undead to save himself, either, but I'm a bit PrCed away from that, too 
To be fair to our Mystic Theurge, he's played in a more tactically-minded fashion than the party wizard, who, for all his incredible cosmic power, fails to get out of harm's way and chews up a lot of my hp, via the wonders of Shield Other.
(Which shall henceforth be cast by our party tank - thankyou Imbue with Spell Ability!) The workmanlike attitude may have something to do with my lack of hostility, though in all honesty, it doesn't take away from my perception of my character's party role to have someone else capable of shedding low-level spells to patch people up. It's actually quite liberating to not have to do all the healing - and there's usually a lot to do in one go, as our DM tends to aggregate the combat into less frequent, more demanding fights.
Well, I'm not surprised by your post. A few humble perceptions:
1. As I've indicated, the main reason for playing the Theurge has less to do with pure power, and more to do with the campaign flavor and role-playing. While we do have a fair amount of combat encounters in our campaign (and they can be longggg fights and brutal), we also have a fair amount of campaign dev, and role-playing, which the DM rewards typically with bonus exp. So, for the min/maxers out there, I certainly appreciate the view that my character will be "weak", however, there are other reasons for playing this character.
2. True, my character won't be able to cast cleric spells past 4th level, limiting his clerical usefulness if you will, but we have no cleric in the party!! So, any bit of additional cleric power (healing and protection) would be welcome. In fact, in this campaign which is less magic oriented than many campaigns, survivability has been a problem to a degree without a pure cleric. We have no ready access to potion shops and the like. So, it is surprising that we have gotten this far without a cleric greater than 1st level (party level is 5-7)
3. Which brings me to my next point. Survivability. The extra hit dice from cleric levels and the prospective 2 fighter levels does help my character's survivability in this campaign. I believe the 5th level wizard we have (pure wizard) has something like 19 hps. It is surprising that he is still alive in this campaign sometimes given our very poor healing capabilities.
4. Not being able to fight in melee. Well, my character has a base STR of 14, and based on my projections, he will be Cleric/Fighter/Wizard 3/2/5 at 10th level, which gives him 2 attacks at that level, only 1 level behind Rogues and Clerics, which isn't bad. With buffing spells like Bull's Strength, he is a fairly effective melee combatant at lower levels, particularly since with a weapon like Spiked Chain, he gets reach and with Imp. Disarm, he is relatively effective in combat. (e.g. smart combat). He isn't a bull like the party tank, but since our Ranger is an archery specialist, at this point we need to have someone else who can partly mix it up a bit besides that party tank (Fighter/Barb). Granted, as levels get higher, his combat usefulness will decrease but hopefully, the increased power of the party wizard will increase to offset some of this. In addition, there are a couple of other spells at higher levels (I think it's "Divine Power"???) which add to combat usefulness if necessary. I'm thinking of the 4th level cleric spell that increases your base attack bonus to your character (at work, no books).
5. Lastly, I read through most of the wizard/sorc. spells in the PH last night. Many of them require touch attacks or ranged touch and have either no save or partial. In addition, most evocation spells have relex saves for 1/2. So, even though my "wizard" efficacy will be less than a pure wizard, there are still ways to affect high level baddies in a "support" role. Again, smart spell selection. Can I stop a high level monster with "Domination" or "Hold Monster" with my less than ideal spell DC's? No. But, then again, the idea of even a 20th level wizard dominating a Pit Fiend is dubious I think with many monsters having increased save DC's. My assumption is that I will be taking SP, and GSP in order to beat SR a fair bit of the time. If I get past the SR, at least I will be able to affect a great many creatures even if they are making saves. I do plan on empowerering many of the evocation damage spells to get the damage cap. up for example.
So, in conclusion, I assume my character is a support or "secondary" character in a great many ways, and I'm all right being a bit of a filler. Sure, it would be nice to be a min/maxer, and have a cleric/fighter/wizard of 1/1/18 at 20th level, but I do have other reasons for wanting to play the MT, and based on most of the responses on this thread, the MT isn't broken. If anything, in many ways, the MT is weaker than maxed out core classes.
Cheers