Some plastics will sustain a flame. Others won't. Not spreading a fire is obviously preferable in almost every circumstance. What advantages do combustible plastics have over non-combustible plastics, and in what circumstances?
Fire retardant fillers are pretty cheap; often cheaper than the base plastic (Alumina Trihydrate for example). Like anything in engineering though, there are always exceptions and functional reasons why something is never the solution 100% of the time.
Here are some instances where I think fire retardant fillers would not be preferred:
- Filler Capacity - A plastic can only accept so much filler. A fire retardant filler would take up this capacity that may be better used for a UV filler or other.
- Elastic Modulus (resistance to deformation)- Fire retardant fillers typically make a plastic more rigid.
- Transparency - Unless you have identical indexes of refraction, any filler will make the plastic opaque.
- Weight - Fire retardant fillers are typically heavier than the plastic.
- Disposal - Fillers may make it more difficult to recycle a plastic or burn it in municipal gassifier or boiler systems.
Also, just because a plastic doesn't sustain combustion by itself, it does not mean that it is safe in a fire. Often there is additional sources of energy that bring the plastic to its smoke point. Smoke inhalation kills as many or more people than direct fire does in situations like plane crashes.
It depends on the plastic and its intended use. Most, but not all plastics will burn, giving off noxious fumes. To make them "flame retardant" increases the cost and in many cases reduces the physical performance properties of the material. Many applications require flame retardant plastic materials, such as aircraft interior parts made of plastics. Look at Underwriters Labs. 94-V ratings tests.