Exposing this information is an important first step. I'm sure we'll discover all sorts of ways this information can be used to improve user experience.
I've been thinking about bandwidth regarding video quality, particularly in the Netflix and YouTube examples, where payload size / quality can be dynamically adjusted. Perhaps, armed with the proposed APIs / headers, we'll begin to apply similar dynamic adjustments to other kinds of data?
For example, when retrieving database records, perhaps it will be common practice to download them in small batches over slow connections, and in larger batches in ideal conditions?