There are shapes you can achieve in 3D printing that you can't with CNC machines. Waste material is a significant problem if you were to have a machine on your desk or your living room.
Another advantage is that 3D printers actually have a community around it and people pushing its boundaries, as well as new companies forming in the space to push the tech forward. That's bound to make it better--or at the very least, explore what's possible. I don't see the same thing happening with CNC machines. No new CNC machine startups that I hear about.