I don't agree with deliberately causing a disability in a child. Ever.
Having said that, though, I can see in the case of deaf parents why they might want only a deaf child. I have deaf friends with hearing children, and I helped to raise my ex's grandson who is deaf, so I've been allowed to see some of the challenges in both situations. (If there are any deaf board members please correct me if I screw up too bad- I'm only speaking from 2nd hand experience here as I am hearing.)
Deaf-from-birth parents (those most likely to try for a designer child) who have a hearing child are in for years of heck. They often can't teach their child to speak because their own speech is not good or possibly not present. They usually can't sing songs or read stories aloud. They can't go to the child's school without an interpretor. If they primarily speak ASL, english may be a second language to them. With the current climate of the US at least, they face years of being "aided" with parenting by friends, family, and schools. They will be treated at times as if they are mentally challenged; they may be reported to Children's Services for refusing to allow "assistance" they do not want.
Dead parents with a deaf child, however, take their child to schools that speak their language. Programs and interactions are things they can all do together. Teachers sign. ASL may considered the primary language. Hearing parents can learn to function in this world- deaf parents cannot learn to have hearing.
Aside from this, at least in America, is the fact that there is a growing political componant to being deaf. Many deaf folks consider themselves different from hearing, but not handicapped in any way. To be "deaf of deaf" (a deaf child of deaf parents) makes you gain respect in some areas of deaf society. When my ex's grandson goes to college at Gallaudet, he will probably get some flack for being "not deaf enough" because he is deaf-of-hearing and able to speak.
My guess is that deaf parents who choose to have a deaf child want a kid who will be a part of their world, just like anyone else. Go to the schools they went to, learn to sign the same songs, face challenges they can help them with. It all depends on how you look at it.