I don't think that's the idea. I've had multiple interviews where I've been asked to code something to show that I know how to do it (even something simple as modifying a website or showing a counter). This just eases that process.
More to the point, showing your skills after you've been granted an interview, and being forced to do spec work before you are allowed to apply (wtf?????) are not the same.
It could be an exercise to build a very simple webapp in 15 minutes. I mean, there are so many 'build a blog in 15 minutes' tutorials out there. It would test if you have enough rails/django/x-framework experience to build a simple crud app.
Besides, this is just a platform, it's up to the interviewer to set an appropriate task. The interviewer can provide an existing web app and ask the user to do something with it, instead of writing something completely from scratch.
