The stem cells have a property that they can turn into any type of cells if required.
So a lot of diseases if they occur, can be tackled by using these stem cells. But its not necessary that the individual will suffer from such a ailment in which the stem cells can be of any use.
You may have heard about stem cell transplant and stuff for lot of diseases like diabetes etc. In such cases, it can come as helpful, but if the child never suffers from any such disease, then you might think that it was a waste spending so much money.
So basically the benefits are a lot, but in the end its your call.