It seems like for ordinary adventuring or if your STR is hilariously low, this is a plausibly useful build. You make up for the accuracy loss of rapid attack with the attack bonus, and STR doesn't matter. Each critical hit is worth 3 damage on average, and doubling that gives you 6 damage. That's really good, and outclasses pretty much any non-rapid attack return on investment. A Deathblade gets that much with 1 lb, 2 STR and Momentum, but gets criticals slightly slower. Overall, though, a Deathblade with 2 STR and Momentum would do almost as much base damage as two attacks from Nargil.
The fact that Nargil has Sharpness is surprisingly unimportant. Instead of hitting full armor with one attack, it hits half of armor with half of an attack, twice. It ought to have some effect, since the Deathblade has more variance in damage, but Nargil can take more shots at getting a low roll on armor, but I'm hoping it doesn't do that much. The effect of the +5 (reduced to +2) accuracy is small, and Deathblades have accuracy bonuses too.
In this sense, Nargil would clearly be better if you can't use a Deathblade that effectively, because STR is too low. But 2 STR is really easy to get from gear or skills or stat buy, and a Deathblade is already comparable in damage to Nargil. But if you add Rapid attack to the Deathblade, and also add potions of STR and DEX, which you have probably have enough of for all important fights, the Deathblade gets twice as effective, while Nargil gets a couple damage points better.
Basically, Nargil is decent for ordinary combat if your other options are unimpressive, or if you want a 0 STR run, and saves you several thousand XP from Momentum and Strength, but is pretty bad when you add potions of STR to the mix, like you would in the throne room or against a high unique or something.
Angrist seems serviceable for this kind of project. You lose a bit of damage, but against the scary high armor monsters you get it back with interest. It requires a lot of Melee skill though. Angrist might even be usable in the throne room, since 2 * 3 * {[(accuracy-evasion)/4.5]+1} is the amount of damage you do.
It's not that good. Deathblades are better. What you want if you want to use a dagger is Angrist. Angrist is actually quite good against things like Balrogs and also more common than Nargil.
Though, you can clear the throne room with only Nargil, subtlety, and rapid attack, I did it once but the morgue isn't on the ladder. Presumably Angrist would be better. It's still probably worse than a deathblade, and you will really want rings of damage <+1>. With a decent enchant deathblade and 2 rings of damage <+1>, you can easily get into 1d15 with 6/crit. With a good slaying bonus you will start seeing 70x2s on Morgoth.
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