The sole reason for selecting t/o flap in a light aircraft is to allow a lower rotate speed, and a correspondingly shorter take-off run. Once airborne, even with only 10 degrees flap selected in a Cessna, the climb performance is degraded slightly as the l/d ratio is slightly worse than it is flap up. Otherwise light aircraft would climb with flap selected.
I've asked a few pilots to show me a short-field take-off on check flights in the past and had them rotate at the normal clean Vr with 'short-field' flap selected.
ST