A head light should be readily available as part of your normal gear. There are plenty of times when both hands will be occupied and in that case, you will be forced to put the penlight in your mouth. Drooling on the patient is universally frowned upon.....

It is perfectly permissible to hand carry a head light, and there are situations where this is preferable, but it is often advantageous to switch from one mode to the other.
Geezer in Chief