Over at Tenkar's Tavern, Eric brought up the perennial debate about whether or not a thief is necessary in an adventuring party.
He makes the point that at low levels, they suck at thieving. In fact, if I recall, someone out there showed how in Moldvay, thieves have a worse chance than any other character at making one of the checks. Never mind that the Thief is not one of the original classes.
Tenkar brought up the the Thief because in one of his parties, they have no thief, and it didn't cause any issue.
Similarly, in my Trelleborg campaign, the thief has had some success, but not enough to make him a key to the party's survival.
One of his abilities is completely hamstrung as I don't allow thieves to pick pockets of party members - the game is open table and I seem to always have at least one new person joining. I am of the opinion that those kind of shenanigans lead to bad blood, especially between strangers. Since they don't really adventure in town - the dungeon is the reason for the campaign in the first place - the thief hasn't tried to pick the pockets of NPCs (i guess the black metal goblin ashigaru demand killing first, stealing items second).
Although he takes scouting missions, it's primarily because he is wearing leather, and the halfling, who could scout as well with their ability to stay still and go unnoticed, plus infravision, is wearing noisy plate mail (she is the party's muscle, despite having a human fighter in the group every other session) and not because he necessarily will go unnoticed.
Although he has had some success on his Thief Skills rolls, a number of those have come when making use of the D30 Rule which greatly limits his risk of failure.
As a general rule, the party smashes doors open, and ditto for wooden chests/boxes/crates. With a magic-user with the right spell list, they could use some more finesse but even then, they could just bring the locked item back to town, try to find the thieve's guild and pay them to open it.
Clearly, the Thief isn't necessary from a game play point of view.
But they do have some advantages.
Chief among them is that some players just want to play that kind of character that they believe would have the Thief skills, or maybe they really like the idea of establishing a thieve's guild as their end-game. It might also be an optimal choice for a player who wants their character to survive.
What? How so?
Reasons the Thief Might Outlast Your Fighter
Everyone knows the thief sucks at low levels, so they are often relegated to the middle or rear of the party at worst.
Most everyone expects them to fail their check for traps (is there any among us who does not recall the fate of Black Dougal?), never mind disabling one they do find, and so parties often don't bother to even have the thief check.
Because, at least in my experience, parties tend to smash containers open when possible, traps in locks are less of an issue as a result.
Where the thief does excel is their improved hear noise, as it's on par with the Elf ability at 1st level, and improves quickly. In Trelleborg, after the thief has performed that check, the fighters/halfling/dwarves force open the door while he is in relative safety behind them.
In combat, because thieves typically have good Dex and thus missile bonuses, they don't have to close ground anytime soon with opponents. If they WANT to close ground they'll probably try to hide in shadows, move silently and then back-stab, doing their best to avoid head to head confrontation.
Finally, the Thief class offers rapid advancement.
The thief in the Trelleborg game is already 4th level, while the next closest PC has just crossed into 3rd. His skills are starting to surpass what just anyone would be able to do, and he can take more than a single hit now which makes him a touch more durable should he fall from a sheer surface (one of the skills at which Thieves excel from the get-go).