That may a case of the dealer thinking "we've always done it this way". Betcha a loonie that if you left the block heater unplugged on a cold night your Prius will start easily the next morning.air4ce said:
In addition to the reasons others have listed: the traction battery (the thing that both drives the motor and serves as a starter for the engine) is rated at a nominal 21 kW; this is approximately equivalent to an ordinary "12V" battery delivering 1600 Amps. At -40C it should still deliver roughly 17 kW (about 1300 Amps equivalent). There should be no trouble cranking and starting.