I'm applying because I want work that's more interesting or more money or who knows what.
Apart from the money, such a "audition project" could help you a great deal in deciding whether this company actually meets your expectations.
You're of course right in that it will be difficult to do it like that for someone who already has a fulltime job.
False dichotomy. I'm already earning money doing real work. I'm not applying so that you can cut into my family time. I'm applying because I want work that's more interesting or more money or who knows what. If you really want to know, ask, instead of assuming ;)
EDIT: Clarifying the "more interesting work" expression.