I only use them as medics because they are so weak there is no danger of them defending first.
If a medic chariot is getting targeted first you are in extremely dire straits. Frankly though you shouldn't be promoting until need said promos anyways, this should leave with the option of making mounted medics at a moments notice.
by doing this (assuming I don't use my garrison troops later when I go rep/US/PS for actual fighting).