I'll some times fire up when the wind is too light. I'm not as maneuverable as some other boats, particularly in light wind, so I end up motoring when I'm in tight spots unless the wind is fair.
That being said, I don't really motor that much - because I'm significantly slower under power than I am with fair winds. My motor moves me at some 3.5kts at 1500 rpm, which is about as fast as I ever like to run the motor. I can run it higher, but I don't get much (I can run at 5kts if I want to consume twice the fuel, and make me fear the health of my 'old isuzu).