The reason they put left hand drive on certain BMX bikes is because of grinding. Some people are more confortable grinding on their right side. But when you mess up and drop off the peg, you land on your sprocket. By moving it to the left side you're less likely to bend your sprockets. By the way, have you seen the size of BMX sprockets lately? They run really small sprockets and even smaller freewheels (9 tooth I believe)?