Hybrids work at their best around town when they can set off under electric power and cover short distances before the engine kicks in. As well as taking power from the engine, energy is also recuperated under braking. The batteries are relatively small compared to a PHEV, but that means a PHEV can travel further under electric power when fully charged. You're right that a PHEV will use more energy than a electric or combustion-engined car due to its extra weight, but it also represents a good compromise for many people and has the ability to travel without any tailpipe emissions. Many PHEV drivers will use their car mainly for short commutes (sub-20 miles) which can be done without using any petrol, but the engine is still on hand if required for a longer journey.