And yes, it has been proposed in the elementary's blueprint to use Mutter in replace of Compiz
On the other hand, Muffin, Cinnamon Shell Window Manager is being proposed as well.
From here, a few perspective is being viewed:
Multi-theme support (dark themes)
More testing is needed
Not much customization as compared to compiz
Less animation possible
No blur effect
Inconsistency with Metacity as fallback
As discussion is being held, what would you think of the switch over Compiz ?
Read the blueprint here