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).