It isn't limited. It would be in addition to a full language. A language that the parents know and use. A language in which they are fluent and can joke and be colorful. A language in which their family would be fluent and the child would be surrounded by users.A language which they can use to explain everything they know. A language in which the parent can go to the zoo and read the signs to their child. A language in which a parent can converse freely and with fluency from day one.
What a horror!
I agree it's horrible, even used in addition, but okay, though perhaps redundant. It's when CS is used alone it can become ugly.