1. We all know what hurts humans and that they are easily injured. They can not sense the other species and are easy prey for them as they fall victim to every spell they weave.
2. Witches/warlocks can do magic. Born witches are stronger in power then ones that just choose to practice. Born witches need no incantations for any spell.(Think The Covenant here) Non born witches need incantations for most spells/charms.(Think Willow and Tara here)