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Drag brake is generally used with 2wd on slippery tracks where trying to apply the brakes manually will often lead to a spin out. At my local indoor track many drivers use a lot of drag brake to slow the car for corners and weight the front wheels for better turn in. Then you have to throttle through the corner to keep the rear of the car from spinning out. As for 4wd I don't find the drag brake to be as neccessary because 4wd brakes much straighter. Also, drag brake and 4wd leads to a lot of nose dive off jumps.

As for run time, I wouldn't expect to see much difference with only 10% drag brake if you are running on a track. What you are probably noticing is that you can drive a bit harder with drag brake because you can hold the throttle on longer into a corner and then the car turns quicker so you can get back on the throttle sooner.