Your "Current Usage" view is brilliant. I'd love to see this kind of snapshot for the ends of each billing period.
Sure, you could drill down "Billing History", choose the billing period, click on the details link in the billing category (voice, text, data), and then choose a device/line. You get a detailed chart of that device's consumption, but you'll need a calculator for a total. And time to do the same for each category for each device.
A link on the "Current Usage" view could show you past usage, quickly, and completely enough for quick comparisons.