Introducing the Electric Duster Project

It seems that most of the conversions I've found mate the motor to a manual transmission and modify so that you don't need to engage the clutch to change gears. Why is this? Your blog states that most of the time you plan on keeping the tranny in second gear. Wouldn't you get greater top end speed and range if you were to shift into third gear? Is there any disadvantage or limitations to using an automatic transmission?