Not necessarily. My PHEV (a RAV4) reserves a portion of its battery to use in the same way that a non-plugin hybrid does. The vehicle maintains a charge in that portion through regenerative braking and the motion of the engine, so the battery can never truly go dead.

Even if I never charged mine, I could still drive it as if it were a standard hybrid and continue to get ~39MPG highway. But in reality I do charge it and most of my miles per month are all electric (most PHEVs also have pressurized fuel tanks to keep gas from going stale when it’s unused for lengths of time).

I drive a Mitsubishi Eclipse Cross PHEV. Highway fuel economy is roughly 6L/100Km (about 30mpg) when driving on a depleted battery. Much better than the 9.5L/100Km (24mpg) from the i30 it replaced.

Not as efficient as a Toyota hybrid, but on daily commutes it uses zero fuel and roughly 20kW/100Km.

So the okay fuel economy is offset by the ability to run on electric for 90% of my driving with no range anxiety.