Sometimes it seems like Apple moves in mysterious ways. Most of the time I can chalk up decisions made in Cupertino to one of two things: Making life easier/simpler or "engineering difficulties."
The iPod touch is Apple's gateway to iOS for those who can't or won't get an iPhone. It's great for kids, for example, or anyone who doesn't need the phone and 4G networking capabilities of the iPhone, but still wants the small form factor. So I'm puzzled as to why Settings (on the latest iteration of the iPod touch running iOS 7) won't allow us to show battery percentages, as you can on the iPhone and iPad.
All I know is this is driving my son crazy, as he knows enough about batteries to know when to worry about the percent (and he knows it isn't an exact science), but prefers a number to a nebulous graphic. Maybe it's OCD or whatever, but the decision to exclude this feature seems to be simplicity for simplicity's sake. I'm hoping a future iOS update adds the feature to the iPod touch.