Although I would tend to be on the pro side of the argument, to play devil's advocate I could some up with a few reasons why it could be detrimental.
One argument that could be made is ASL does not use the same grammatical structure that typical English does. In this respect, a child being taught ASL could have problems with sentence structure and grammar, having a hard time learning that there are differences between the two languages and also mixing up the structures.
Another argument against using ASL would be the feasibility. Adding an additional subject to students studies could be overwhelming, And even if the child/student can handle the work load, will the child ever use it. Unless the child is in contact with people who use ASL it would seem like a waste of time. If the child is not constantly using the language it will be forgotten. So not having contact with ASL speaking people will only allow classroom or little room for conversation, leading to loss of the language and therefore a waste of time.