The point of the W3C (they are the ones writing the standard in the first place) is that the current draft is not complete and the problem with implementing now based on browser implementations is that in the future, you might introduce standard inconsistencies, where you're using parts of the draft that never made it to the standard or parts that were modified but that browsers implemented early and of course, wrongly.
Also, right now, a lot of HTML5 CSS properties are still browser dependant, using the moz- and webkit- prefixes. When browsers drop the prefixes, you will have to update your CSS.
Basically, the W3C is advising against using the draft in its current state. They should know what they are talking about.