Wow. This is horrible advice. I'm sure it works for the hiring company to get a better picture of the candidate, but this is unethical and potentially illegal in some states/countries. Legal issues are why many companies recommend/require references in a specific format- 'this person worked here from [date] to [date]', etc... devoid of any 'opinion' statements.

Here is a per-state summary of job reference shield laws (sorry, not sure how current it is): http://www.crimcheck.com/resources/job-reference-shield-laws...