They're requesting the ability for anyone to extend the standard without risking incompatibility with future versions—in exactly the same way that anyone can already extend XHTML 1.0 and XHTML5.
Sam was appointed co-chair (with Chris Wilson of Microsoft) of the HTML working group in January 2009, and I expect he'll be a prominent supporter of some version of this proposal. But the primary editor of HTML5 (Ian Hickson of Google; his co-editor is David Baron of Apple) has so far preferred more limited additions that address specific use cases, such as the "data-" attributes in current drafts, or hard-coded support for specific W3C vocabularies like SVG. His reasons are similar to yours; he hopes that by forcing all extensions to go through the HTML working group, the W3C might prevent implementors from fragmenting HTML into mutually incompatible dialects.
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They're requesting the ability for anyone to extend the standard without risking incompatibility with future versions—in exactly the same way that anyone can already extend XHTML 1.0 and XHTML5.
It's not just Microsoft. Sam Ruby of IBM has been requesting it since 2007 when he made a very similar proposal: http://intertwingly.net/blog/2007/08/02/HTML5-and-Distribute...
Sam was appointed co-chair (with Chris Wilson of Microsoft) of the HTML working group in January 2009, and I expect he'll be a prominent supporter of some version of this proposal. But the primary editor of HTML5 (Ian Hickson of Google; his co-editor is David Baron of Apple) has so far preferred more limited additions that address specific use cases, such as the "data-" attributes in current drafts, or hard-coded support for specific W3C vocabularies like SVG. His reasons are similar to yours; he hopes that by forcing all extensions to go through the HTML working group, the W3C might prevent implementors from fragmenting HTML into mutually incompatible dialects.