A rogue, specialized in subtlety, is likely to attack with daggers. When Burning Crusade was released, everyone avoided daggers, but ever since the community of rogues lamented the fact that daggers wre not powerful enough, Blizzard increased their effectiveness. Once the daggers had been the ultimate weapon for rogues and with Wrath of the Lich King they slowly regain their position. That is very fortunate because some rogue attacks like mutilate deep in the assassination tree need daggers to function.

Some other rogue attacks that require daggers have the additional prerequisite that the rogue must be in a position behind the target. In raids this in seldom a problem since all melee units position themselves behind the bosses anyway lest they get hit by the frontal attack. And besides, bosses cannot defend against attacks from behind which would reduce the damage. Unfortunately there are also bosses which are immune to this. When dealing with Thaddius in Naxxramas, for example, the rogue will stand right in front of the boss when he is as electrically charged as the main tank. The same goes for the solo game where the enemies will confront the rogue with frontal attacks as well. That is why the assassination spec is not as popular as the combat spec which allows for full benefits no matter where the rogue stands. But despite theses disadvantages, assassination is still a much used raid spec because of mutilate which just does a ton of damage.