From HTML WG Wiki
Something for Everyone, Not Everything For Anyone
The primary markup language of the World Wide Web should aim to extend the range of communication and make the web more accessible, universal, and inclusive. This however, does not equate to one size always fitting everyone. Technology provides a great opportunity of personalization and customization of content.
Two quotes sum this idea up:
accessibility is something for everyone, not everything for anyone, it's the content that's important not the interface - Adrian Higginbotham
The rationale for personalisation for accessibility is clear. Accessibility is essentially dealing with diversity - personalisation avoids the always flawed attempt of doing so by a "one size fits all" solution. - Martyn Cooper
The longdesc attribute is a mechanism that makes this idea a reality. It affords users a method to receive information in accordance to their needs and capabilities. Because one size doesn't always fit all, HTML5 should not obsolete features if not all users can fully make use of them but rather alternative/equivalent mechanisms like longdesc should be provided. Ensuring access to information for those who would otherwise be locked out and lose their opportunity to use the Web is the socially responsible and right thing to do.