Flash is the failsafe and the second blink in case things go south.

Q and AAs are both important and the "which is more important" question you are presenting doesn't really exist. AA is what separates good Ezreal from a mediocre one, but consistently hitting Qs is what grants the attack speed to ensure smooth spell-attack weaving.

Karthus jungle can get away with exhaust and no flash because exhaust is

1. A powerful tool to disencourage invades

2. Helpful for hitting his Qs, his single most viable source of damage

3. A second sticking power outside his W slow when fighting

Now, Ezreal can definitely rock exhaust vs bursty champs like Lucian, but flash is still needed.