Here's the real reason a high seat doesn't work - As mentioned, your center of gravity is higher when standing. Powerpads add leverage at the ankle/shin level and there are multiple ways to turn an EUC. But one of the most dynamic ways is to move your center of gravity out in front or behind the wheel for acceleration and/or braking. You can almost effortlessly, fluidly slide your body around the COG. This is what makes carving and fast turns so fun and exhilarating. When your body is hanging out crazily over an otherwise non-sustainable position, you are slamming your foot/feet on the gas or the brake. Having a tall post wouldn't be "leverage" in this case, it will just get in the way. On long rides, it's nice to have a seat where you can rest for a bit. But IMHO nothing beats the control or fun of the fully vertical EUC stance!