The Inverted Router offers many advantages when cutting plastic, such as:
The material is sandwiched between the template and the table which helps to stabilize the part for less chatter marks in the edge.
Removal of chips through the dust-collection system (working from below) helps prevent chips from welding back to the workpiece. A better cut means less preparation before flame polishing.
Simplicity -- templates for the Inverted Router are far simpler than other conventional routers.
No programming is required, no special holding fixtures, as on a CNC router.
Construction holes are drilled during the router operation for a perfect fit.
Most cutting of plastic requires a simple cut-out operation. If the job can be cut as fast, or in many cases faster, with an Inverted Router why use an expensive CNC?