I never feel the difference in throwing wpf really, nor the throwing speed. It is just a requirement for pt I think. I have tried throwing with 1 wpf and 0 pt in mercenaries mod. Not much difference but the damage.
That's always the way I felt. It didn't seem to matter how much WPF I had in throwing, my weapon's always threw in the same spot of my crosshair, it's just a matter of getting used to the weapon...
But I've had others (pure throwers like Chucky) who swear by going max WPF for accuracy, but also for extra damage. They have a lot of PT too they need to make up WPF drop for, but I know there's people out there who swear by having high WPF in throwing....I still think it's pointless though.
IMO there's no need for pure throwers, you're much better off going melee + throwing.
*EDIT* The guy above me (NJ_Legion_Icedtea) reminded me of the main reason Chucky (or some other thrower) told me they use high WPF, it's because they can get their weapon primed to throw much faster than with lower WPF.