Theoretically, burnable discs can deteriorate in time if exposed to light.
In fact, the writable substrate is light sensitive, so theoretically keeping it away from light should be sufficient to avoid deterioration. The data surface, however, will deteriorate anyway with time, but a good quality disc will take many years to become a coaster. This is indeed the main drawback of low quality media: they can become coasters because of the dye deterioration after a short time (some media can become coasters in few months).
The only drawback of plastic bags is that often to keep out the disc you can scratch it, but if you are careful this "side-effect" will be minimal
About the second question, burnable discs are different than pressed discs. Manufacturing process is different, and whereas in burnable discs there is an organic substance that can be written by a laser beam, in pressed discs the land and pits are created by a different process (pressing using a mold) and are not writable.
Moreover, using the press method, is possible to make things not possible with a burner (some games or audio CDs have protections not replicable with a burner for this reason). I don't know details of this however :o