I would not have an abortion if I was told, while pregnant, that my baby would be disabled, either mentally or physically.
My niece, Gabrielle, has Wolf-Hirschhorn Syndrome, which is a partial deletion of the fourth chromosome. My brother and his wife were told about this about 3-4 months before the due date, and while they prepared as much as possible for whatever could happen--by studying the disorder, talking to doctors, etc--it was difficult because WHS is such a rare disorder that there isn't much information available.
The thing is, there are different ranges of disability with WHS. Physical and mental development can be severely delayed, or only mildly delayed, or anywhere in between. The child could live for fifty years, or it could die within a few days. (On that note, I want to add that we celebrated Gabrielle's first birthday a couple weeks ago
)You can't know how disabled your baby will be until well after he or she is born.
That would be my main problem with aborting a disabled baby. You might be expecting severe retardation and decide to have an abortion, but what about the chance (and often a very big chance) that the child might come out with only a clubbed foot, or deafness, or something.