Plastic is a generic term covering a wide range of materials, it is a good insulator and this helps to keep the mouthpiece cooler. Some plastics can easily be produced in a wide variety shapes and colours including opaque, versicolour and marble effect patterns. And so it is great for pimping your device and general purpose use. POM, PEI and epoxy resin thermoplastics are recommended for sub ohm or high power vaping. With all plastics care should be taken not to drop it onto a hard surface with weight attached to it (your tank or mod) as it is liable to break. However this might well save your much more expensive equipment from damage.