I like the "service animal" analogy. But it's not that easy to escape the ethical fork/tradeoff, as the breeding of universal companions would be far better resourced than that of service animals. Then it's not "only some people are allowed to have cell phones", it's more "we'll ban cell phones and if someone really really has needs, we'll let them have a milk-carton brick of a radio telephone", with an implicit "and that will be good enough for them".