Quint is powerful, but not OP. See, if you use Quintessence, you need to spend a card space on that. That means the creature you're gonna quint better be worth it. Most popular choices are creatures that can grow damage, such as Firefly Queen, Lava Destroyer and Spirits.
The Immaterial creatures Immortal and Phase Dragon have no useful skills, and so they only do damage. Would you quint a Dragon solely for 8 damage? I wouldn't. I would pack another Dragon before a Quint. So, Phase Dragon simply lets you save that card space if you want that.
This means you have to be able to stall a bit, because Immaterial creatures are a -lot- slower than normal ones. If you can't stall, you'll get rushed.