There's a benefit from a publisher's view point to focusing on a single standard.
If HTML5 gets similar market share to Flash, I imagine in the long run most content publisher will choose the one that's better supported and easier to implement rather than maintain support for both - for the same reason that many now choose only Flash, rather than Flash+Silverlight+whatever.
If HTML5 gets similar market share to Flash, I imagine in the long run most content publisher will choose the one that's better supported and easier to implement rather than maintain support for both - for the same reason that many now choose only Flash, rather than Flash+Silverlight+whatever.