"Is dive certification really necessary?"
Legally necessary? No, not in the vast majority of political subdivisions around the world.
Do smart people get trained? Yes. Emphatically yes.
Do smart people need certification even though they have been trained by a competent individual?
No. Depending on the diving a person does, certification is not necessary at all.
If
@Akimbo trained me to dive, I wouldn't give a crap about having a c-card.
Except, for my type of diving, there is a strong agency bias against non-conformity. A diver who deviates in attitude, gear, and gear set-up is given the stink eye.
I must have certification that exceeds the planned diving that we are accomplishing in order to avoid negative attitudes from my juniors.
cheers,
m