Can a character with the Defensive Duelist feat who never makes an attack with the finesse weapon still gain the benefit of the feat?

The description of the Defensive Duelist feat says (PHB, p. 165):

When you are wielding a finesse weapon with which you are proficient and another creature hits you with a melee attack, you can use your reaction to add your proficiency bonus to your AC for that attack, potentially causing the attack to miss you.

Assume a spellcaster who has the Defensive Duelist feat is holding a dagger and an arcane focus. For the duration of combat, they cast spells and never make a melee weapon attack. Do they still gain the benefit of the feat?