Great question. In my opinion its the single target execute alongside the power of dizzying swing.
Dual wield is in a good place in pvp, lots of people use it well too.
2h = burst
Dw = constant damage
I'd even say DW is better in duels like on a stam sorc or something. It all depends on the skills on the bar and the player and opponent.
It sounds like you believe DW is bad, and its really not that bad. Especially if you got a maelstrom dagger or two.
DW could work in PvP for NB, because they have a gap closer (ambush), a cheap spammable (surprise attack) and an execute (killer's blade) as stamina morph class abilities and a fear effect from mass hysteria. Other classes need the knockdown from dizzying swing and the execute from executioner. But rally can still be useful to a NB since it is the only way to get major brutality without breaking stealth - power extraction has to hit an enemy to grant the buff.