It's not stating that group has less value, it is stating that that group has less ability to think which is just as true as blind people have less ability to see, or physically disabled people like me have less ability to walk than people with normal feet.
That doesn't inherently make someone less valuable, and my point when using such a term as blind or retarded directed towards a non-disabled person isn't to attack their value but instead to attack their behavior, specifically to point out that they aren't utilizing their abilities to their full extent. If you hear the word retarded and think that means the person is less valuable, maybe check your assumptions about disabled people's value.