While I can certainly see the merits of a "blessed" codec for HTML5, I think that the endorsement of a particular codec is bad engineering practice, unless the goal of HTML5 is "presentation of video" (which as far as I understand it, isn't).
It's goal is to present modern webpages, webpages which increasingly rely on video, for better or for worse, specifying an open codec would ensure that users don't get left out due to using the wrong browser/OS/media player, as happens frequently now.
HTML4 has been around for a long time, and I think this is partly because it is open and flexible, and does not tie anyone to any particular implementation of anything. For the most part, it's done a nice job distinguishing what you must have, but not what you can have.
But videos are now becoming more of a "must have" than a "can have" to many people. This would make it possible for the average user (as in, someone who has no idea what a "codec" is, or for that matter, what "flash" is) to watch videos without havign to worry about getting codecs or flash, just install the browser and you have support for one or two standard formats.
I'm not saying OGG should be the only format used, or that browsers shouldn't rely on OS libraries or anything, which many people seem to think is being argued, but it'd be nice to have the assurances that browsers will always support at least the one open format, so web designers can use that and not worry about their users not supporting it.