Partially agree. A good hiring process can boost your odds of finding the “right” people, but my personal experience has been that firing well is at least as important as hiring well. Firing well is also something that most companies do very poorly.
Once proven that a candidate is able to learn and adapt to the job, they should be moved to a full-time position, with pay above what is currently being given to that role. If they aren't excellent, let them go. As a company you want to advertise this promotion experience so that people who know they will pass this test will feel more comfortable signing up for a "lowly" temp position.
The top 5% or so of candidates might not take that offer, but you can leave a hole for exceptional candidates based on past experiences and proven success.