>Sending a ping on leaving the page is a pretty contrived example

I don't think it's a contrived example. It seems to be one of the main use cases. From the spec:

> The main use case for the Beacon API is to send analytics such as client-side events or session data to the server. Historically, websites have used XMLHttpRequest for this, but browsers do not guarantee to send these asynchronous requests in some circumstances (for example, if the page is about to be unloaded). To combat this, websites have resorted to various techniques, such as making the request synchronous, that have a bad effect on responsiveness. Because beacon requests are both asynchronous and guaranteed to be sent, they combine good performance characteristics and reliability.