Someone always suggests this. The issue is that if you did this, 95% of the candidates that would agree to this kind of setup would be the kind you didn't want.

If I'm sitting on 2 offers, one is a hire and one is a "Let's see how this works out after 2 weeks of work", I'm going to take the first one. And that says nothing for the necessary benefits question in the states, where changing jobs often involves expensive (in money or time) changes to health care insurance.