[DougWebb]

I'm in the US. My data plan is capped when I'm on my carrier's network, and unlimited (and much more expensive) when I'm roaming. Typical US carrier.

This highlights a good point: it's not just the number of bytes of data, it's the cost per byte. Many data plans have an explicit cost per byte, but even ones that don't will have an opportunity cost: if your app is sucking up a portion of my bandwidth, you're preventing my other apps from retrieving data. Even if my data was free, my bandwidth is limited.