But if the site used prefixed css, it's possible that no other browser would render the site properly anyway (many sites these days are made with just chrome/safari in mind)
The only developers who make sites that work just in Chrome or Safari are rank amateurs. No professional would do such a thing. Professionals understand that a web site must work for all visitors and I have never heard of any web site that would do otherwise. To me, this is only an internet rumor and not based in reality.
The only exception would be when one particular feature is needed to display one specific, unique element. Off the top of my head, I can't think of what that would be.
The goal is to discourage browser-specific content in a way that doesn't result in users switching browsers.