The ideal companion to a somewhat greedy shelf electric power, AC allows a miniUSB Y-cable for charging the tablet. Besides I have more than one large miniUSB cable has to carry everything (memory card reader, external hard drive, portable DVD burner and the microUSB devices like phone and tablet)
Otherwise some tasty tablets can continue to discharge (speed) even has a USB port connected because the power per USB port is limited to 2.5W (5V and 0.5A).