Does it matter "what we can expect" from persons with disabilities?

Here is an interesting exchange (read also the comments) between Peter Singer (Princeton) and Michael Berube (Penn State), whose child has Down's Syndrome.  The exchange grew out of Berube's blogged comments about a recent conference called "Cognitive Disability:  A Challenge to Moral Philosophy."    Berube writes, to Singer:

You’re looking for things people with Down syndrome can’t do, and I’m looking for things they can.  We each have our reasons, of course.  But I don’t accept the premise that cognitive capacity is a useful criterion for reading some people out of the human community, any more than you would accept the premise that we should grant rights to animals on the basis of whether humans think they do or don’t taste good with barbeque sauce.

This comment, from "Father of Tommy", struck me:

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. . . .

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?