Quote:
A big part of the need for an energy resource is undoubtedly the dodge mechanic; players can roll swiftly in any direction they choose by double-tapping a movement key. This is a huge element in both making combat more dynamic and placing responsibility for each player’s health in their own hands; unlike the Rock ‘em Sock ‘em Robots combat model, in Guild Wars 2 all damage will be avoidable. With this formidable new capability comes an equally formidable need to limit its use, else rolling becomes the most attractive combat option for all modes of play – binding skill usage to a resource that is depleted by 5% every time you roll is one sound way to accomplish this goal.
http://www.killtenrats.com/2010/12/03/gw2-energy-blast/