Having recently spent time with AD&D 2nd Ed and Labyrinth Lord made me realize just how pointless the thief class was. Any character using their ability score to do anything would be more successful than a low level thief. A wizard with a 7 Str is more likely to climb. A cleric with a 9 Dex is more likely to hide in shadows or move silently.
But if a DM doesn't allow ability scores to be used, the thief has like 15% chance to scout ahead and do his job.
Even though I have fond memories of these games and they have their place in gaming history, they are poorly designed.
I would disagree with some of them being poorly designed (of course, that depends on how one defines designed). It's made for a playstyle that differs from the ones today.
The thief is an interesting class in that it wasn't originally planned for. Originally you had a Fighting Man, a Magic-User, and a class that combined the two (magic-user and fighting man, but maybe not as great as either) in the Cleric. You could play the entire campaign with just these three classes. A DM could figure how they climbed over a wall, snuck past the dragon, figured out where the traps were, and other things that were later relegated to a Thief. Many chose to use ability score checks.
When the thief came around, the original usage suggested was that unless under stress, the thief would automatically succeed in their Thief Skill checks (at least that's how I understand the original creator used their thief class, he's still around as well, perhaps even on these forums though I do not know his handle/name here). I don't think Gygax was as thrilled with the implementation in quite that way, so I do not think this idea was communicated in any rules of the day. He had the thief, but in some ways it was pared down tremendously from that type of usage.
If you use it as many did where the thief gets a DOUBLE CHECK, which means they can make all the ability checks as others do, but also can fall back on their thief skills, or use their thief skills but also get an ability check pertinent to them for a lesser success, it actually gives the thief a greater usage. Many who chose to do this had been using the original 3 classes with ability checks to do things already, so using a thief this way made a lot of sense.
Not everyone played it this way (why I said many...not most or all), and there were various takes and interpretations of how the rules would work.
[and if you consider that poorly designed, I suppose you could be right, but I don't think that's what you meant. For some of us, that type of design works better than the modern rules that define everything you must do in a game].
When AD&D hit the market, no wording of this was really hinted at. Those who played with the Older groups who started early only learned about these things and how this worked via word of mouth. It was further killed off by interpretations at tournaments which for some crazy reason had weird minded judges that came up with excuses as to why a Fighter might not be able to climb up a cliff, or why they couldn't hide behind a pillar (only a thief can hide behind a pillar...let's ignore common sense). AD&D was much more strict on rules and specific types of rule interpretations. As AD&D got older you no longer saw Fighters sneaking around as much or Magic-Users using their brains to solve puzzles with no combat or magic (as magic to low level Magic-users was limited). Later AD&D basically had the thief being played as you described originally and AD&D 2e continued this tradition as did 3e.
I don't consider any of those rules really bad design though. I think that's a throwaway word (and they tried to use it with 4e in reference to older editions quite a bit, I think that put a bad taste in a LOT of people's mouths in regards to that consideration) to try to promote newer editions and insult older players and their rules.
I think it is a DIFFERENT type of design that caters to different playstyles and how different groups enjoy playing. For some types of playstyles it's perfect, for others, it is not.
5e I think tried to take some of this idea (easier to homerule, less stress on being absolute rules and exact rules for everything) because it saw the strength of this type of design that was found in earlier editions.
