Local dad spoke only Klingon to child for three years

I don't know what I can say about this.

I know that a child will develop its intonation at the early age and will be very difficult to change later in life. I myself is a good example of speaking broken Cantonese-toned English. I also know that a child can simultaneously master several different languages almost effortlessly at the early years.

However, is it best for the child to teach her a language which has very limited real life use?