Knowing how to defend yourself against dark wizards and any other threat out there is compulsory. It's not about how heroic it may seem when you perform these spells to save yourself or others. It's about what you do to defend yourself. If the spell is performed incorrectly then your life and possibly others is at risk.

Expelliarmus -
Disarms your opponent

Stupefy -
Stuns your opponent

Lumos -
Ignites the tip of your wand.