Well it is the latest standard.
It will probably be several more years before HTML5 becomes a standard and by then a lot of the proposed changes will be shown to not work in the way that was expected and they will either be changed to something different or removed.
After all IE5 supported the CSS2 proposal at the time it was released and by the time CSS2 became a standard there had been massive changes made to it - the cause of all the CSS problems with IE5/6/7 today - too many people creating live pages using a preliminary draft rather than just testing with it.