If the crank is out of unbalance, it will vibrate at high rpm. People (such as myself) balance a crank to reduce unwanted stresses on internal components due to things whirling around out of balance. It also reduces friction and can enable a bit more power to reach the wheels. A lot of people run A-series without balancing the cranks and they claim to still rev them over 7000rpm. If I was building a decent road machine, I don't know if I'd bother balancing an A-series.