The way I look at it is that yes, the extension can be useful for requesting a particular file format (IMO the Accept header is not particularly accessible, especially if you are just a regular web browser user). But if you have a default/canonical representation, then you should give that representation in response to a URL that has no extension. And when you link to that document in a representation-neutral way, you should link without the extension.
That doesn't stop you from also serving that same content from a URL that includes the extension that describes the default/canonical representation. And people who want to link to you and ensure they get a particular representation can use the extension in their links. But someone who doesn't care, and just wants the document in whatever format the website owner recommends, should be able to get it without needing to know the extension. For those situations, the extension is an implementation detail that is irrelevant to most visitors.