In-game stat window for assassin says the following
Strength=Melee damage bonus,Melee Crit chance
80str=+16 melee damage bonus, +0.91% crit chance
Willpower=Melee damage bonus,Melee Crit Chance, Force Damage bonus,Force Crit Chance
565= +113 melee damage bonus, 5.90% Melee Crit chance, +113 Force damage bonus, 5.90% Force crit chance
And as far as i can will power is increasing damage. Str helps melee to but seeing assassin has both force attacks and melee might as well get willpower stats. (tested with out armor and armor on Str is same with and with out and will power does increase melee atlest that what it looks like to me)
If this is wrong then the in game description on assassin is wrong on what helps with melee but, Str probably helps more, but i dont have str gear to test that. might want make some changes to the main post about willpower not helping melee