Actually rather than 5V, a standard should be established that can be delivered easily with some multiple of cells... such as 3, 6, 12 or some such, so for the need of totally portable power one could resort to batteries. 5V is pretty arbitrary these days... held over from TTL. Current cell phone tech is running on 4.5 and 3.2V...

Any voltage can be regulated to deliver what ever a device needs, so why not use a 1.5V cell multiple as a starting point.