I have tried this and find it to be extremely tiring. Trying to hold the gas on with only thumb and forefinger is a strain and takes away fine throttle control. The tenth of a second lost in transitioning from full grip to brake lever is minimal when you know ahead of time that the brake must be applied. The car that turns in front of you didn't do so in a tenth of a second and slowing for the possibility will more than make up for the lag. There is another, more fundamental reason, though.
Motorcycles are air-cooled, which means they are cooled by the flow of air over the engine.
Most motorcycles don't have the large radiators cars do, nor can they sit for any length of time idling. My Gold Wing has a small water supply that is circulated over the engine, and a tiny fan that turns on when the engine is hot, but it is not nearly as efficient as a car. When the fan turns on it is usually a signal that the rider had better find a clear stretch of road to get the engine cooled down. The alternative is to pull over to the side and shut down, letting the motor cool. Even slow movement, say through traffic, might be enough to keep the engine cool, and stopping will certainly help it to seize it up.