Plastics may be selected to be machined in low volumes or for some prototypes .

When a prototype needs to:

• Exhibit product material properties (color, finish, strength, optical clarity, etc)
• Be tested for physical performance (load capacity, fatigue, temperature/chem resistance, etc)
• Requires high quality dimensional accuracy that cannot be achieved with rapid prototyping.

This link has some good information for selecting a plastic suitable for machining.

Ref: Connecticut plastics.