Plastic is not per se worse than other materials. Cheap aluminum, as Lenovo uses on cheap Yoga and ThinkBook laptops, is definitely not a better choice. Those devices do not use the expensive CNC aluminum of more premium designs. They use aluminum as a trick to make it seem like a high quality design when it isn't.
Advantages of plastic:
- Cheaper
- More robust (because it flexes slightly, which means it does not break as quickly)
- Transparent to radio waves (which is very advantageous for a device with LTE)
- Not as heat conductive
The main disadvantage is that plastic feels cheaper, but that is merely perception and does not mean it is actually worse quality. And also, robust plastic designs tend to be thicker, which is also the case here.
To avoid it feeling cheap, Lenovo uses the rubbery-like coating. Plus, this device has an internal metal frame made out of magnesium, so it doesn't flex as much as a pure plastic-construction.