There are two ways to approach this question, and I think the answer depends upon which you take.
Personally, my cousin has Downs Syndrome and, as MKECharlie said, he brings a lot of smiles to everyone and generally lives a happy, fulfilling life.
From the other point of view, however, he can only do so because he is being financially cared for by my aunt and uncle. He will never be able to provide for himself nor contribute meaningfully to society.
So, I have a feeling that Dawkins is taking the second point of view, while the rest of us take the first. Speaking as an economist, yes, I agree with Dawson. Speaking as a person, no, there are many things wrong with that idea. Personally, though, I would rather not have the burden of taking care of a child with a mental handicap. However, I applaud those who are strong enough to do so.