What can we expect (and does it matter what we can expect) from people with disabilities?

Here is a fascinating exchange (read also the comments) between Peter Singer (Princeton) and Michael Berube, whose child has Down's Syndrome.  This comment, from "Father of Tommy", seems especially important:

Why should our expectations of others be a criterion for moral judgment as to their rights? Is it that our expectations are some kind of reliable indicator of what others are capable of? As your examples show, that is likely false, particularly in instances where theorizers are trying to come up with reasons for killing other human beings.

Why don’t we actually look at what they are capable of, rather than sit in our offices and seminar rooms talking about what we expect of them?

But then why should what they are actually capable of be a criterion for moral judgment as to their rights? Do we recognize disabilities? To recognize a disability tacitly recognizes that an individual of a certain kind is suffering. It is not a disability for a worm not to be able to see. But if one recognizes the suffering of another human being, why then not do what one can to alleviate the suffering, rather than destroy the sufferer?