vee41: Good points too.
I'm not sure the community on steam can be qualified as casual (but there is a potential debate on the term casual that isn't really the point of the topic).
But indeed, steam represents a large audience and tools to marketize.
To temper my previous post, what really bothers me is some people expecting/talking about a steam integration for C2 like "the end of all pains".
At least I believe that's how they see it. And it is the case one has to be aware that steam isn't the ultimate answer.
You still have to struggle for visibility amongst thousand of other products. And you can't really customize/tune the appearance/package of your game to your liking (all games are displayed the same in the steam shop).
I agree it's tricky/hard to get people to your website (though it can be done).
The advantage is that once the user visits your website, "you won", you're not fighting for visibility, you're all there.
The user doesn't have to watch a video or screenshot, or download a huge client/soft, install it, configure/execute it.
With your HTML5 game, he just goes to the page of your game and once the download is done, he's playing. And also, as the application can be made offline, he won't download it again next time he comes to the page.
I'm OK to have a potential steam integration (also keeping in mind that Steam does not make its API public, if your game is accepted on their platform, than you have access to the documentation of the API, so a plugin that potentialy could exist in any sold copies of C2 is unlikely due to Steam's policy itself), but I do not see it as anything critical or required to monetize right now.
It's another possibility/tool, not an end in itself.