Re: iOS 8.1.3 is out now. Post feedback here.
I think I follow your point on the potentially misleading aspect. If someone uses Facebook all day, then by extension Facebook app should show up as a relatively high battery usage %. Same for Home & Lock Screen since to use an app, you have to access the Home Screen.
The Battery Usage screen has helper text that says "Shows proportion of battery used by each app when iPhone is not charging." So perhaps as an example, if someone doesn't realize that they are using Facebook all day, then there could be surprise to see Facebook app listed high up in the list.
Battery Usage for an app is not a bad thing, since the only normal way for an app to use battery is to use the app. Now if an app shows up in the list that has not been used, or used infrequently but has a proportionally high % usage - then I think that's a reason for concern.
Before the Battery Usage feature came out, we were left to more or less guess if there was a rouge app taking up excessive battery. I caught few apps doing this only because they were leaving crash logs. I was able to deduce after a few times that when a particular app (native Mail app on iOS 7.x) crashed, it wouldn't let go of the CPU (per the logs), and in turn took battery life with it.
I'm glad the Battery Usage feature is there.