Most of the engineers that I've hired came from another job (not all though), and so they need a confirmed offer before they are willing to jump ship.
So this practice introduces its own selection bias; you'd only be sampling from the pool of currently un-/fun-employed, and those who hate their current job enough to quit without a safe landing.
I'd vastly prefer to have a week or two of paid contract work from each candidate before making the decision, but it feels insulting to even ask.