WWP is one reason, and the glaive is a second no doubt.
Remember though that Dark Eldar are essentially bullies. If your opponent has a nasty combat HQ character - shoot him!
Archon and Succubus are both great for chopping their way through MEQ's, but put them up against anything with a 2+ save and they will have more problems (yes the succubus can get through them, but against either a real combat character, who'll have a good backup invun or a unit of 5+ models all with 2+ armour she won't have enough on her own to take them down)
Haemies are still important HQ's as they bring the master of pain rule, so can really boost survivability of the unit early in the game.