You made it sound like the best professionals only blame themselves and don't give any objective answers regarding what could've been better with who (or what, in terms of policy/procedure) is remaining behind as they leave.

There's a middle ground for sure. It's obnoxious when people don't take any responsibility and only seek to blame. That being said, the blame game usually starts long before the exit interview — that's almost always a character flaw that exhibits itself early. So if you know that's how the employee behaves normally, brush it off. If it's not their norm, perhaps, when criticism is given, some introspection should be had on your part to know if it's all just sour grapes, or if there's validity to their complaints.