I came across this link recently, and of course being a cold, heartless philosopher, much like Singer, I immediately began to think about the philosophical issues involved. The author claims that Singer's main mistake is in believing that disabilities reduce quality of life. It is not completely clear whether she thinks no evaluation can be made of another person's quality of life, or if she only thinks most people's judgments about disabilities specifically are mistaken.
If she takes the former position, then I find that view quite implausible. Further, I find the implications of the view very troubling. It would seem to leave us without tools for evaluating public policy. Surely the goal of many government programs is to make people better off. And the goal of many laws is to prevent us from making one another worse off. If we have no basis at all for making judgments about whether people are better or worse off, how can we devise these policies and laws?
On the other hand, if she does not wish to make the general point, but only wishes to make this claim about disabilities, then I have a different set of reactions. First, it then strikes me that this is an empirical question. Whatever tools we use to evaluate how well off people are generally, apply them to particular disabilities and see what results we get. Second, the thesis that disabilities do not make people worse off still seems to have some unlikely consequences. It suggests, for example, that there is no reason to put any effort into curing a disability; if there's a line of research which could conceivably repair spinal damage and so cure paralysis, it would seem on this assumption that we have no reason to fund such research, since it wouldn't make anyone better off.
I'm sure Singer presented arguments like this. I wonder if there is any response to them.