Face cards are just high value words - they get removed from the deck during character creation so that you don't start with all your most powerful cards (if you were lucky) and have room for your grimoire to become more powerful as well as more flexible. But if you have ideas for additional effects, absolutely add them in!
I thought using them as summoning spells could be interesting. Jacks could summon trickster or mischief entities. Kings and queens, depending on the suit, could summon high ranking fae, or various deities, all up to the paradigm of the wizard they're taken from. They'd only be spells acquired from wizards, not from your own grimoire. Just an idea.