They significantly reduce primary energy usage. When you generate one 1kWh in a thermal power plant you also generate an additional 1 - 2 kWH of thermal energy. So in statistics it will look like the thermal power plant produces more energy. If your city has district heating it might be a valid form of power, but right now we assume all the thermal energy is always useful and necessary which is especially bad in cars where you turn up to 70% of the energy into waste heat.

By increasing energy efficiency by a factor of 2 you easily cut down on energy usage.

Nuclear power plants rarely have been used for district heating. Lot's of nuclear power plants are heating up rivers...