Throwing weapons are strange. They say they do Melee DPS, but in fact they contribute to Ranged DPS(you can test this by checking your ranged DPS numbers with it equipped/unequipped.
This happens because all ranged attacks do either Ranged DPS or Spell DPS depending on weapon(look at your Basic Ranged Attack tooltip).
The small increase in Melee DPS you get from having a Throwing Weapon equipped is from the +Melee stat on the throwing weapon. Throwing Weapons do not contribute to Melee DPS like equipped Melee weapons do.
The main reasons to have a Throwing Weapon equipped are to use Jolt(which is a ranged skill that calculates off Melee DPS and requires a throwing weapon) or to gain the benefit from several talents that require an equipped Throwing Weapon(Steadfast, for instance).