David Smith implemented iOS 7’s background fetching in his Check the Weather app, with surprising results:
When enabled within your applications you are essentially building a massively distributed botnet. Each copy of your application will be periodically awoken and sent on a mission to seek and assimilate internet content with only the OS safeguards holding it back. As your app grows in popularity this can lead to some rather significant increases in activity.
Based on the data Smith shares, “rather significant” is a “rather significant” understatement. The world of background data fetching is a potentially expensive one.