OK, OK, yes. Of course imagine my chagrin after digging through my closet to find my old PHB only to remember that I lent it to a friend several years ago!
But whether we're talking about 10% hide in shadows or 25% for open locks, my point still stands that a 75+% FAILURE rate results in a character who isn't very useful overall.
Compare that to a Fighter's combat effectiveness, or a cleric's 100% success rate when casting Cure Light Wounds, and the thief starts to look like the bastard step-child that he is.
I also found it amusing that MAGIC was the LEAST variable and MOST reliable element in the game.
While I agree with your premise, I want to point out; clerics with wisdom less than 13 suffer SPELL FAILURE when casting their spells. Granted, its small (20% for wis 9) but it does remind us all that for a game that prided itself on "3d6" bell-curves, your really weren't any good unless you had AT minimum a 13, and realistically a 16 (and most often an 18; come to think of it, I never saw a PC with less than an 18 in prime) prime requisite.
I will agree though that most thief skills were too low. Again, you needed an 18 dex (with its +10 to most skills, give or take) to make them usable at 1st level. Most of the time, low-level thieves were a.) Multi-classed with fighter or magic-user to make them useful or b.) Snipers with a bow until they got some magical items and levels.
Still, with an 18, a 40% PP, 40 OL, 25% F/RT, 25 MS, 20 HS, 10 HN, 85 CW, and no RL was acceptable. Just make sure you
fudged rolled that 18 Dex!
However, backstab was a real waste of time though. First, you had to a.) not be noticed by the foe so, lets pull out that 25/20% success HS/MS rolls, b.) hit, though a +4 was nice, it was usually negated by a thieves less than 17 strengh and 2nd worst thac0; and c.) all for... 1extra dice at low-level, 4 extra at high. Assuming you used a longsword, you did 2d8 damage (+ whatever magical plus, which was NOT doubled). Compared to a fighter whose 18% strength granted him a guaranteed +3-+6 damage per hit (plus spec, if used) and backstab looks kinda pathetic. And that's not EVEN comparing how much easier and more effective a spell from a M-U is!