Backdoor references are very unethical in my opinion. Beyond the fact that you are telling a third party about someone’s job search without the searcher’s consent, you can also get a very biased opinion.
But frontdoor references are guaranteed to be biased no one (well, almost no one) is going to use someone that didn't like them as a reference, they are going to dig up someone that will say something good about them.
We had one DBA hire that was not very good, she lasted 3 months before we had to let her go because she just couldn't do her job. One of her references didn't have much good or bad to say, but her most recent manager (where she worked for 5 years), gave a very glowing reference, outlining all of the projects that she had helped with.
It wasn't until we had let her go that we found that that while the person that gave her a glowing reference actually was her manager, she was also her roomate... and her girlfriend.