I work in ED now, working on my FNP. All of the nurses that were ED NP had to go back for a semester or so and take the test for FNP. Im not sure if that is how it is everywhere but at this hospital they want them to be "family" certified because they will see the most stable patients, which are generally clinic type patients a lot of the time.
I don't think that it is a wasted degree, I see them hiring more and more because there is a shortage of ED doctors. If you later decide you don't want to work in ED and you have the FNP you could always go just about anywhere.
Just my 2 cents.