Yeah I think it's worth noting that not only was backstabbing very difficult to perform in AD&D, with lots of caveats involved in it's use. There's a lot of misunderstanding about how it really worked, so let's take a closer look.
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So our first caveats are that your Thief must surprise their foe, and attack from behind. Their foe must be unaware of their presence. The target must be humanoid in shape, and the Thief must be able to reach a vital organ they can strike. Only the first attack made can be a Backstab.
The attack is made at +4 to hit, but it should be noted that this replaces the normal +2 to hit any character gains from a rear attack. Left unsaid is if this includes the +1 to hit gained against a surprised opponent.
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The loss of Dexterity bonuses is part and parcel of a rear attack (and also of a surprise attack). It's also probably a wash as few monsters have a listed Dexterity.
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Ok, but +2 to hit over anyone else is great, right? Not so much.
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Sure, Rogues level up faster than Warriors, but let's compare a 7th level Fighter and their equivalent (64,000 xp)...a level 7 Thief! For this exercise we'll put them both at 70,000 xp, making our Thief level 8.
This gives our Fighter a Thac0 of 14 and the Thief a Thac0 of 17. With their special bonus, the Thief has a Thac0 two better than the Fighter. The Fighter theoretically has better bonuses to hit from Strength, but rather than worry about that, we'll mention a bonus a Fighter is much more likely to have- weapon specialization for a +1 to hit. So we can see that our Fighter is 10% more likely to hit a foe under these conditions than the Thief.
Ok, but what about that "one big hit"? Well, as you can see from the section above, the only damage that is multiplied is the damage of the base weapon, not magic or Strength or anything else. So, let's look at the best weapon a Thief can natively use- the Broadsword (because again, Backstab against large-sized foes is much more difficult).
2d4 x 3 for our 7th level Thief is going to be 15 damage on average. Our Fighter is probably using a longsword because, well, it's AD&D (and honestly the Thief should be as well, but that's another whole discussion).
His attack damage is merely 6.5 (including weapon specialization). But he has more than one attack! In fact, thanks to his level and specialization, he now has potentially two attacks per round! That brings his damage to 13 (and again, we're not even getting into potential Strength bonuses!). And yes, the Fighter can make these attacks in a surprise round as follows:
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Sure, there's two weapon Fighting, but both classes can engage in this if they so choose. So ok, maybe we got off 2 extra damage with all our rigamarole- but it doesn't stop there!
Remember, we need more than surprise- we need our foe to be unaware of the attack! That means we probably had to succeed at Move Silently and hope our opponent doesn't turn around. And bear in mind, we have to move more slowly to use this ability! Hide in Shadows can only be used if we have areas of cover or concealment we can move between- if our opponent has a torch out and is in the center of a room, good luck! We cannot use Hide in Shadows in darkness (go see the obscure rules thread, lol) and any opponent with infravision wouldn't care about darkness or shadows anyways. But it gets even worse!
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Even if we are successful, there is only a 50% chance our opponent is surprised. Less if they have a listed Dexterity! And some monsters are surprised even less often than 3 in 6!
Now a lot of DM's give the Thief surprise just by sneaking up on someone, but it turns out, that's against the rules!
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What this means is, if you set up an ambush and attack first, your opponent
is not surprised because they don't check for that until after you attack! If you attack during an ambush your opponent is now very aware of you, so you presumably have to use your ambush turn to move into position and hope your 50% chance to surprise the enemy comes up in your favor! And worse, if both you and the Fighter ambush enemies, our 7th level Fighter can attack
four times in the span you can backstab once, making his damage blow yours out of the water.
Even if you have proficiency in a heavier damage weapon to backstab with. 2d6 for a greatspear x3 is still only 21 damage, and our ambushing Fighter with his longsword/shortsword combo is doing 33 damage in the same amount of time with just weapon specialization!