Since Manifest most frequently affects the cards on top of your Library, it's actually a surprisingly cheap means of gaining card advantage.
How so? For instance, the aura cycle that manifests a body to attach to. Even if you end up with a creature that you can flip over, you're not really up a card since you're still using the card to manifest it. If you're opponent were to kill that creature you'd still be even on resources.
Yeah but you still get a creatur. Also there are a lot of spells that will put the unuseable cards back in your hand. There are also four Morphs from KoT that get so much better if you manifest them.
It will see constructed play.