Celebrim
Legend
We can certainly agree to disagree, but balance is not a subjective thing. How balanced things need to be to be good enough, or how important you want balance to be to design are subjective things. But balance itself is objective. You can measure it in various ways.
One of the fundamental problems the thief class faces, is that if you try to define its role in the party, what you almost immediately run into is the problem that even if the whole adventure concerns what you would assume would be the thief's schtick, it turns out there are almost no points in the thieves progression that you wouldn't prefer a different class with the same XP. That is to say, if your scenario involves something like, "There is a corridor with a 10' deep pit trap in it.", then generally speaking you'd be better off with a fighter than a thief. Or if your scenario involves something like, "There is a trap filled tomb", which you'd think would be the thieves forte, it turns out at you'd actually prefer when solving that problem to have another cleric with Find Traps, Augury, Divination, Commune, and so forth, or another M-U with a wand of secret door detection, spider climb, fly spells, and a variety of divination spells. Not only does the other class solve this scenario more effectively and more reliably, but in all the other scenarios you might encounter, the alternative PC vastly exceeds the thief in competence at handling that scenario as well. This might in theory be offset by the thief's accelerated advancement, but it turns out that the amount of advancement the thief receives is rarely nearly enough to be balanced. For example, you might take a 2nd level thief in some cases over a 1st level fighter, but that only lasts for 800XP.
One of the fundamental problems the thief class faces, is that if you try to define its role in the party, what you almost immediately run into is the problem that even if the whole adventure concerns what you would assume would be the thief's schtick, it turns out there are almost no points in the thieves progression that you wouldn't prefer a different class with the same XP. That is to say, if your scenario involves something like, "There is a corridor with a 10' deep pit trap in it.", then generally speaking you'd be better off with a fighter than a thief. Or if your scenario involves something like, "There is a trap filled tomb", which you'd think would be the thieves forte, it turns out at you'd actually prefer when solving that problem to have another cleric with Find Traps, Augury, Divination, Commune, and so forth, or another M-U with a wand of secret door detection, spider climb, fly spells, and a variety of divination spells. Not only does the other class solve this scenario more effectively and more reliably, but in all the other scenarios you might encounter, the alternative PC vastly exceeds the thief in competence at handling that scenario as well. This might in theory be offset by the thief's accelerated advancement, but it turns out that the amount of advancement the thief receives is rarely nearly enough to be balanced. For example, you might take a 2nd level thief in some cases over a 1st level fighter, but that only lasts for 800XP.