Well, as I have worked in the PR for a big assosciation of service providers for people with disabilites, I can shed some light into the sexual assistance.
These services have some uses beside the obvious one (which, in Austria, is limited to handjobs, as the workers there are not prostitutes, but actually quite highly educated in paedagogics and healthcare).
Many people born with mental disabilites have never been properly educated in that matter. Thus, it helps keeping unwanted pregnancies down. Also, it's still not uncommon for people with disabilities to be sexually abused, because they're "easy to get". Assistance helps doubly, because it fulfills their need and educates them. In addition, it prevents people from being frauded by prostitutes or pimps.
Also, for many people with disabilities, it's quite hard to actually find a prostitute (which is in general legal here anyway) to carry out their work on them, let alone find a real partner, thus it's not only the only way for them to get that need satisfied, but also it prevents people with disabilities raping others, because the need grew too strong.