This is why it needs to be a challenge related to the employer, but not an actual feature they're going to go and use.
At my last place we used one, said "please don't spend more than 5 hours on it" - it was a challenge to read a specific industry format (e.g. test you can go and find something to read it, not implement a library to do so ... not reinventing the wheel) and present it in a basic RESTful web app (testing that you know what REST is, and can implement basic CRUD).
Never had an issue getting it done, and it was very illuminating, some people who passed through the initial interview with flying colours turned in absolute crap.