Sure, they’re not as fear-inspiring as your good-old two-handers. Their damage output looks kinda pathetic, in fact. But there are two important things: 1) They have an AP cost of only 2. Their damage might suck, but they can keep it up all day. 2) They’re the only weapons that can backstab (by default).
As long as you keep your dagger at about your level, the dagger is surprisingly effective. In fact, if you asked me what weapon to specialize in if I wanted to do huge amounts of damage, I’d pick the dagger over a 2H sword. Daggers AP cost is half that of a 2H weapon. If you save up 14 AP and are next to an enemy, you can attack 3 times with a 2H weapon, but 7 times with a dagger.
You might object that you have to use some AP to move into position, but not always. With a wizard who knows Teleport, and the right turn order, you can save AP up for a turn and move your rogue right behind an enemy with full AP.
Just keep your daggers up to date. I bought a normal-damage level 5 Push Dagger and it was pretty effective, even against the undead (who have piercing resistance).
The one big problem with Daggers is that the skill "Assassin: Backstab with any weapon" exists, which I think steals the role of daggers too much.
Unfortunately, I fear that a nerf is coming, partly because of my experience with The Best Dagger Ever.
The Best Dagger Ever
I found a hilarious magic push dagger (Brutal push Dagger of Storms) that dropped from a boss (PP), which did 32-52 Air damage a hit. It only had one modifier: AP: 1
Yes, that means that it only costs one AP to attack with it.
I faced the Caesar Crab boss. Roderick my mage and Scarlett my Rogue saved all their AP on their first turn, leaving Jahan and Madora to damage the crab on turn 1. I don’t remember how much HP it had to start out with (I think 686, maybe), but I know that after the turn was over, it only had 450 left. Then I had Roderick Teleport a fully-charged Scarlett to the back of the crab. I didn’t get it quite right, she had to burn one of her 15 AP to get into position, leaving her with 14. I couldn’t see how much damage she did per strike, but you’ll get the idea:
She killed the crab… and still had EIGHT Action points reserved at the end. So an average of 75 damage a hit. With better positioning of the teleport, and 15 AP saved up… that could have done ~1125 damage in one turn - and this was Air damage, which is resisted by fewer enemies.
I was level eight when I found it. That dagger only required level 6 and 3 points into Single-handed weapons.
I get the feeling that after a couple patches, daggers like that won't be around anymore. Specifically, I bet the "AP: 1" modifier will be gone.