A skill needs to have weapon damage in order to use attack damage converted to health (exception if this is built in into the skill, like Devouring Swarm). With 100% weapon damage it will use it in its entirety, doesn’t go over 100% weapon damage and gets diminished returns the less weapon damage it has.
People mostly use it for the cast speed and vitality damage.
Bloody Pox can gain a bit of Weapon Damage trough items.
Do note, that “lifesteal” will be penaltized if % Weapon Damage on skill is lower than 100%. For example, if you have 20% lifesteal and a skill with 20% Weapon Damage, you will heal for 4% from damage dealt by its “weapon damage” component (20% from 20%).