Q:

If the output from a webapp can't be made accessible, is it worth it to make the rest of the webapp accessible?

If I have a webapp that cannot be fully accessible to visually impaired users because the end goal is for a non-accessible animate bitmap canvas to be output, is it worth it to make the rest of the webapp accessible to them? If so, what should I prioritize adding?

A:

A non-exhaustive list of visual impairments that would not hinder people from being able to consume movie-ish content.

Colour blindness
Low visual acuity; 

vision that is somewhere between 20/70 and 20/400 after correction (legal blindness in the USA starts at 20/200 after correction)
or a visual field of 20 degrees of less (this counts as legal blindness in the USA)

Blindness;

vision that is worse than 20/400 after correction
or a visual field of 10 degrees or less

Glaucoma; The vision loss is typically tunnel-like, with intact vision in the center, which decreases as the vision loss in the peripheries expands.
Macular Degeneration; A level of increasing opacity forms in the center of the vision. It blurs the sharp, central vision, but does not necessarily result in total vision loss, so may not stop consumption of movie-ish content to begin with.
Cataracts; does not stop the consumption of movie-ish content, even if seeing/reading the content is harder. In fact, movie-ish content may be easier for many sufferers than reading.

Obviously the level of impairment is a major factor; e.g. Cataracts can get bad enough that they cannot see.
Design considerations for these users would allow many of them to continue to use your service. They would also improve the usability for non-impaired users.
Accessibility considerations should also be given to those with motor impairments; they may not be able to easily tap or click on small target areas.
There are also cognitive impairments that should be considered
A former colleague of mine has created a tool that helps to understand many of the accessibility considerations that will affect your product.
http://www.antigravity-systems.com/labs/accsim/index.php

