Actually, using the actors skin can be a great way to ensure consistancy with regard to exposure.
If you have ever worked on a film set with known actors, you will find that you will never ask them to hold a grey card, so, you must find out where on the zone system there skin tones are. Once done, this is usually a great way to meter as your not constantly sticking a meter (or a card in front of the actors face) Incidentally,
do you think the founder of the zone system used a grey card for his metering? No he did not! Ansel used his spot meter to determine where he wanted certain zones to fall and then developed his film accordingly.
There are many great cinematographers that use nothing but spot meters and without grey cards.
And another note regarding this subject-- using the spot meter on African American faces is a great way to ensure consistany exposure. I have worked on two feature films ( as the Gaffer) Barbershop 1 and 2 and here you have a true range of darker skin tones and you must light and expose them with care.