Why, you might ask. Actually its simple.
The thief class grafts a ersatz "whiff" prone skill system into the game that works against the core concepts of the game. These concepts namely that 1st level guys are highly capable already and that level is a measure of relative combat and magic power not skill are an important part of the designs intent.
Most of the versions of D&D up to second either ignored skills or used some variant of the "stat roll" such as a proficiency system to resolve conflicts.
This meant a 1st level character could have as much as a 90% chance of success in many common tasks. Even adventuring tasks such as surprise or hear noise often had a 33% percent or more chance of success
Contrast the thief skills, class abilities really, all those except for climb were far less generous.
Labyrinth Lord thieves are quite pathetic at less than 20% in many abilities. For a core ability this stinks. Less than 1 time in 5 you will make your roll.On top of that progress is fairly glacial, abilities never getting much above 50% till the class hits middle levels.
In 2nd edition, which is fairly generous amounts started around 30% and went up from there. Its not as bad but its still a kludge.
In fact think this grafted on nature occurred to the designers late in the life of 2nd edition as there was an attempt during the Ravenloft 2e years to make some of the abilities into proficiencies. I can't comment much on how that played but it is telling.
I am now coming around to the idea that thief ought to be a background trait rather than a class. Instead of taking thief as a class, a person could say roll against dex to pick a pocket if they had an appropriate background.
This also has a nice side effect for pulpier games. If you re-skin the cleric as a white mage, it leads to some interesting concepts.
One could have a "fighter" with a high dex whose background is "cat burglar" , a magic user who was a grave robber and a "cleric" (now white mage) who is a tomb raider and has mastered some spells to aid in his career.
Each would be a kind of thief with different abilities but none of them would be a thief class
The only real flaw I can see is with existing subsystems for surprise and hearing noise. These would need a way by which they could go up as the character leveled. If that could be remedied the thief class could be done away with nicely.
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