If I level a Dragoon from 1-70 and then pick up Sage, am I supposed to know how to heal effectively right away?
This isn't a player problem, it's a game design problem. It allows players to pick up new roles and jobs at a higher level without having to go through the learning experience to get there.
This isn't necessarily a "get good" situation.
Also, the exact same thing could be said about our OP here. He's asking about basic tanking 101 questions after doing a L70+ dungeon.
You're right about having a responsibility to learn the jobs and roles, but you really shouldn't be jumping to conclusions about a player's experience just because they're on a higher level character.
Edit: A work around for this in the future could be to have a system in place that prevents you from unlocking a new job unless you already have another job of the same role at the unlock level requirement. (Ie. Must have a healer at L70 to unlock Sage)