EDIT@above: Flash cannon is not a situational move. There is not one pokemon that it is better against than another move, nor is there a certain group of important pokemon it hits where you would need two moves otherwise.
That's like saying you should never use Brick Break on Scizor, because Superpower hits things the same coverage-wise, and is stronger. Actually, it's not like that, but w/e.
How about we all just agree that Latios is a better "all-purpose dragon" and Hydreigon should only be used when your team would benefit from its specific assets (such as Flamethrower, not-Pursuit weak, Earth Power, etc.). If you're not using it for those things, then you should be using Latios.