It's about balance. Your probably thinking of against an AI, while I'm thinking of against another player.
Chargers are easy to kill. If all of your chargers are dead... well your deck is useless. The dragons have more life than Armagio and have a nice punch (8 power), so their a stronger shield and can hit back while shielding. Even if your Chargers were wiped out you've probably taken a good chunk out of your opponent anyway and can finish him off with dragons. Also, momentum creatures don't attack a creature that's gravity pulled. That means the dragons, or otyughs, are the ones who have to knock out a pesky enemy creature.