It's not entirely true that they are (daggers) melee weapons, the first skill is melee ranged. The life steal is actually short range (longer than melee, 600). The third skill is a great bit to nail a mob in place, and, again, short range (300) still longer than melee, but shorter than the Life Steal's 600. The first off-hand skill goes to 1200 (blinds, and transfers conditions to the target), and then second is at 900 (bleeding and enfeebling).
A Dagger/Dagger Necromancer has a lot of options for survival, damage and conditions. With traits you have a lot more options to choose from; like the ability to increase survival, lifesteal amounts, additional lifesteal sources, cooldown speeds, movement speeds and others. The proper traits and gear matter a lot in the long run, but it's wholly possible and not in any way weak or useless as I've seen a few suggest.