Galaxie is right, wind does affect your truck if its sitting outside. You will notice a major differance because the engine block has cold air blowing on it when the block heater is trying to heat the engine. In some cases, wind tends to minimize the effectivness of a block heater. If you park were your truck is protected at least a little from the elements your truck will be much appreciative. Subzero temps are hard on every part of a vehicle.
For the guy who is wondering about storing a vehicle...I definately wouldn't worry about leaving any type of heater pluged in the whole time you have it in storage if that is what your wondering. As long as its not running and the coolant is mixed right so it wont freeze( about the only thing you will need to watch for) it will be absolutely perfect when you come to start it in the spring. I have relatives that go south for the winter and leave their wagon outdoors in the driveway all winter long. NO heater, no nothing. Just park it and forget about it. Been doing that every winter for the past 10 years.