If you are looking to impart information, then usability and accessibility are important, absolutely.
Sometimes, I want to read the W3C specs, an essay or a news story. In those cases, I don't want to have to wade through incomprehensible navigation and wait for things to download.
Sometimes, I want to lose myself in a site and just marvel at the wonder of it. I want to push buttons and see what happens; I just want to play with a site.
Usability isn't an empirical issue, it can't be measured. Jakob is talking out of his **** there. Usability requirements are usually worked out from use cases: what does the visitor to the site want to do? If they want news, they have one set of usability requirements; if they want to play with Flash toys, they have another.
Accessibility is another matter. I code to standards, I write standards compliant templates for friends, I an currently working on a site for a government agency which requires that the site comply with WAI-AA guidelines. Yet, I can't esacape the conclusion that accessibility should not be the overriding concern of everyone making a website. Your Flash developer/designer is setting out to make something that fulfills a certain need to make something interactive, playful and/or mystifying. Folks, SVG just isn't even close to being capable yet. So what if it's text based, how is a screen reader supposed to cope with:
There are certain people who just can't use certain things. I don't see people getting so up in arms about DVDs without closed captions or roller-coasters that don't allow people below a certain height to ride.
<linearGradient id="pencil" x1="0%" y1="0%" x2="100%" y2="22%"
<stop offset="0" style="stop-color:#ffe401;stop-opacity:1"/>
<stop offset="1" style="stop-color:#e0c858"/>
If you are creating something that is a visual experience, then you can't hope for it to be accessible to all, the same way a painter can't. Do you really think Joshua Davis, for example, wants to exclude people from seeing the things he makes? But he knows Flash. The problem (such as it is) lies with Macromedia.