I think the GP's point was that if they are looking to get a job, they probably has many options of good places to work (even if perhaps not as good as yours) which would not require this of him, and therefore your audition can be seen as extra work which they do not need.

Now, if I were Very Interested in working at Discourse, Fog Creek, or any similar place of work, rather than just at any place with a nice paycheck, I think the audition is a great idea, and something people would be willing to do.

The GP's main objection is that you only get the best people who want to work at __your__ company enough to spend time on your auditions. There are likely many engineers like them who have a network of job opportunities compelling enough that the added draw of it being Stack Overflow, Discourse, or Fog Creek is not enough to merit the extra work.