THANK YOU!
One of the biggest things people notoriously forget is how convoluted the thief skills in B/AD&D are. Nearly every Thief Skill has so many provisions, riders and quid pro quos that they are nearly impossible to use RAW. That creates a situation where a thief's abilities are only as useful as the DM was willing to allow and had knowledge of the intricacies of the rules. Good DMs knew the rules and applied them in a way that was fair to the Thief. Bad DMs neither knew nor cared and made the penalty for failure the worst possible consequence.
Now it is totally fair that most people did not play with all the rules- either from not knowing them, or their DM's deciding that the rules were sometimes too obtuse (or both).
It's perfectly ok to say "at my table, Thieves were backstabbing machines", and if that's your experience, I hope you had fun! But the
intent of the Thief class was, apparently, to pretty much suck, and I've posted quite a few of my rants about that on ENWorld. People who think Thieves were incredible were engaging in one of four things-
A) not playing the game as written. Again,
no shame in that! I don't think anyone ever played D&D as written before 3e. Even the people who made the darned game didn't!
B) excessive optimization. With sufficiently min/maxed ability scores, the right race, proficiencies, and Kits, there were ways to get better performance out of the Thief. One of the Forgotten Realms books (Warriors & Rogues of the Realms, I believe) has several Kits that give you bonus points for thieving abilities, let you backstab with ranged weapons, and other ways to get around the limitations of the class.
C) happenstance or DM fiat allowing you to get items that either enhanced your abilities, or bypassed them completely. Who needs Climb Walls when you own Gauntlets of Swimming and Climbing? A Ring of Invisibility (and Inaudibility) makes sneaking about a snap!
D) skilled players navigating around the edges of the game's rules. Using mundane items and ingenuity (aka "MacGyvering") to make it so that you can achieve the results of finding traps, opening locks, etc. etc. without ever needing to make a check.