I suppose you could make the debate it's the most convenient source, but your daily intake of regular carbs will be more than enough to supply the brain with energy.
If sugar had no adverse affects, then cool, go wild with it. But it's not as simple as calories in vs calories out. Sugar is high in fructose, which doesn't occur naturally in the body. It can only be metobolised by the liver. A high sugar diet overloads the liver and causes a bunch of health problems. It's also a leading contributor to bad cholesterol. It also causes a dopamine release in the brain, making it highly addictive. If you have strong will power, great. But most don't and over indulge in sugary foods/drinks.