The term didn't bother me because I just adjusted for when the show was made. Certainly if it was used today, it would bother me. Maybe I'm confusing posts but I didn't think the term bothered him, but the misguided termination of a life did.
Perhaps it was debatable then, but certainly not now. I have just seen too many smiling faces on the downs kids to think their lives aren't worthwhile. Even back then, the testimony in the court revealed they were very loved by their mothers and Klugman's DA revealed he loved his downs brother...
A more recent series, The Closer, which had some very somber shows, had a few excellent funny episodes. It's hard for to image E.G. Marshall being funny. Remember, "Dying is easy, comedy is hard."
The Defenders was a socially conscious show and with not only changes in medical advances but in that of our attitude towards the disabled, there will be "dated" attitudes in the show. I cannot remember what my attitude towards a Downs Syndrome child would have been when the show first aired...