[SixOfSpades]

It is, in fact, technically possible to run through the entire game without a Thief, otherwise it would be impossible to solo any other type of character. But it's a genuine pain in the patootie to do so, and only experienced gamers, who have memorized the location of every single Trap in the game, would attempt it.

I have never said that a Thief is an absolure necessity. I have, however, (frequently) said that no strong party should be without one.

Your options:
1) Nalia. Hardly creditable as a Thief on her own (for one thing, her Find Traps score is just enough to make you think you're safe, and then of course you step right on a Trap), the only way you can truly get by with Nalia is to give her a load of Thief equipment, which of course impinges on her effectiveness as a Mage.
2) Imoen. Believe it or not, Imoen has the skills to be all the Thief you'll ever need. Note that I said need, not want. She'll never be a kickass Backstabbing fool, she'll never lay scores of Traps for your enemies to stumble into, she'll never be able to pierce Improved Invisiblity without casting a spell, and the only way you can make a good Stealth scout out of her is to load her down with a bunch of equipment. But she does have nearly 100% in the two skills that every party most needs: Detect Traps and Open Locks. If the Traps aren't gonna nail you, and 95% of doors and chests will open for you, you're home free.
3) Jan. The fact that he's Multiclassed, and therefore gains Thieving points more slowly than a pure Thief, is of no consequence when you consider that his starting equipment boosts his skills to the point where you (practically) no longer even have to worry about how to get Jan's skills up to 100%--you only have to worry about which item to wean Jan off of next. His combat skills aren't much behind a regular Thief's, and his Mage spells of course add quite the punch.
4) Yoshimo. I don't care for the Bounty Hunter myself, since they're lacking combat-wise compared to the Assassin and Swashbuckler, and of course combat is pretty much what the game's all about. Some players, though, love them, perhaps due to the cheesy abuse that their Special Snares can be used during a fight, by "throwing" them from outside the enemy's sight range. And, of course, there's Yoshimo's Chapter 4 activities to be considered. Personally, I either carry him along if I'm in a hurry to get to Imoen, otherwise I immediately drop him for Jan.
5) PC Thief. A very popular choice, particularly interesting when it comes to such Duals as Assassin->Mage, Swashbuckler->Mage, and Swashbuckler->Fighter. (And, of course, Kensai->Thief and Wizard Slayer->Thief.) You must be wary, though, about Dualing: If you stay a Thief long enough to get 100% in more than four skills, it's going to take a long time to get your Thief half back, and that means a long time struggling with Locks and Traps. Multiclassed Thieves, of course, don't have this problem.

My observations:
100% in any skill is enough. You might want to go as high as 125% in the Stealth skills, however, since such factors as bright light act as negative modifiers to your score. And you should never Pickpocket with a score less than 240% anyway.
Half of a Thief is enough Thief. Heck, even a quarter of a Thief is enough, in Imoen's case.