I'd love to see more research about the benefits of bilingualism (or multilingualism) for those who are Hoh or deaf. Also regarding the dichotomy of mulitlingualism being encouraged in the hearing world, yet discouraged for those who are hoh/deaf (in terms of visual language)
For years a horrifically large percentage of Audiologists (including 3 of my own over the years) were formally taught, and then regurgitated the myth that if a Hoh or deaf child/adult learned sign language (or any manual communication) it would negatively impact their speech and listening skills ... however this is simply NOT true.
In the hearing world, bilingualism and multi-lingualism are known to be positive attributes which increase intelligent and linguistic comprehension - thus encouraged in most, if not all countries (there are a great many countries in which it is perfectly normal for people to grow up fluent, or nearly so in 2 or more languages ).
Thus there is a massive inconsistency in the idea that hoh or deaf individuals may somehow be harmed my learning another language, and will somehow use a specific language, or type of language as a "crutch". This is specially an "invented concern" in terms of visual languages which are so naturally suited to those who's hearing loss making comprehending speech difficult, if not impossible.