What is the Purpose of a Service Dog?

Today I was reading a debate on the Assistance Dog Group hosted Yahoo Groups on the necessicity of task training for service dogs in order to be considered a service dog. First, it is not the dog that has any rights; it is the person with a disability as defined in the Americans with Disabilities Act(ADA). [Further discussion on the definition of disability across the legal feild] A sevice dog as currently defined under the ADA is:

Q: What is a service animal?

A: The ADA defines a service animal as any guide dog, signal dog, or other animal individually trained to provide assistance to an individual with a disability. If they meet this definition, animals are considered service animals under the ADA regardless of whether they have been licensed or certified by a state or local government. Service animals perform some of the functions and tasks that the individual with a disability cannot perform for him or herself. "Seeing eye dogs" are one type of service animal, used by some individuals who are blind. This is the type of service animal with which most people are familiar. But there are service animals that assist persons with other kinds of disabilities in their day-to-day activities. Some examples include:

_____Alerting persons with hearing impairments to sounds.

_____Pulling wheelchairs or carrying and picking up things for persons with mobility impairments.

_____Assisting persons with mobility impairments with balance.

Further definitions from From Miriam-Webster Online regarding "service" and "Assist" since service dogs are also sometimes referred to as assistance animals:

Main Entry:
1ser·vice
Pronunciation:
\ˈsər-vəs\
Function:
noun
1 a: the occupation or function
of serving active service> b: employment as a servant 2
a: the work performed by one that serves b: help, use, benefit be of service> c: contribution to the welfare of others d: disposal
for use

Main Entry:
1as·sist
Pronunciation:
\ə-ˈsist\
Function:
verb
transitive verb : to give
usually supplementary support or aid to intransitive verb 1 : to give support or aid 2 : to be present as a spectator

The keys here are that the person have a disability the "substantially limits one or more of life's major functions" and that the dog in question has been trained to
perform tasks that the person cannot perform themselves and that mitigates their
disability. The argument on the list was in regards to what happens if the person's disability doesn't need any of the commonly trained tasks? In that case do you
really need a service dog? Based on the aforementioned definitions the answer is a
resounding no. If you have a disability that you feel would be mitigated by having a service dog, you should also be able to identify ways the dog beyond their mere presence can lessen the affects of your disability. It is important to remember that service dogs are not the latest in accessory. They are highly trained[see the Training Standards from Assistance Dogs International], working animals that enable their handlers to accomplish tasks and interact with in the public domain in ways the disability without the presence of the equiptment, namely the service dog, becomes unreasonably difficult or impossible.If a person cannot identify any asks for which a dog can be trained that will mitigate their disability beyond the
comfort of their mere persence, then they may want is an Emotional Support Animal. The only two places Emotional Support Animals are granted access under the laws are housing and flying.