Burning plastics produce large quantities of highly toxic dioxins. Apart from that, incineration is an expensive and inefficient process.
In general terms, recycling waste plastic is far better in environmental terms than incinerating it. When you incinerate plastic, you are left with a good deal of residue (called bottom ash), and this contains toxic substances - mainly Cadmium and lead. levels of Cadmium in bottom ash are generally twice the limit permitted for landfill disposal, and that creates a big problem - where do you put huge quantities of toxic ash?
So no, incinerating plastics as a means of generating electricity isn't viable.