Acceleration Nation is perhaps a misleading title. Achieving great adrenaline-generating speeds is rarely the goal. You're heading off to the cinema, to work, to a café, for gods sake. But the bicycle needs to be set into motion and often standing is a lovely way to do it.
Accerlating on proper, upright bicycles still allows you remain upright. You find yourself pulling on the handlebars rather than leaning forward as the bicycle is propelled forward and your centre of gravity is stable. Plus it just looks nice.