Lots of companies have policies that dictate only the most qualified applicants will be hired. If you want to start throwing personalities into the mix then there will be litigation from the "most qualified" person when they aren't hired.
Race card, sexism, you name it. This is the world we live in.
Personality is not a protected class.
Personality is a large component if an individual is qualified. Do you want to hire a Project Manager who may have their PMI Certification, MBA, etc to find out that person doesn't have a strong enough personality to manage a project team of strong personalities? If the most qualified person, interrupted the interview with a phone call from his headhunter about another position and took the call in front of you, would you hire them? And yes, I've had this happen.
And in regards to legal issues, you are under much more legal scrutiny trying to terminate an employee than hiring one.
In this economy, I can find dozens of people that are qualified for the position. The most qualified person is also going to be the individual that also fits into the company's culture, team dynamics, enthusiasm, eagerness to learn and be a quick learner.
And like I've already stated, you can't ask an applicant for previous employers reviews. The more information I can gather to make an informed decision the better.
Companies perform drug testing, google your name on the internet, run a credit report and other background checks but that still doesn't mean that one is going to be a good fit for a position.