For me, the simplicity, appearance, and sound of the air-cooled engines are what do it. I'm sure that liquid-cooled is much more effective (especially if you want to compare it to a late-model air-cooled engine, after they shaved the fins down by 50%). However, the kind of riding I do generally involves higher average speeds and much lower average RPMs than motocross, so I simply don't need the extra cooling power. The bike never gets particularly hot, why add moving parts and pump liquid to the top of the bike for cooling I don't need? For someone making a serious bid at motocross racing victory, however, the situation is different.