I think this is a critical feature if Bitbucket is to be a true alternative to Github for open source initiatives. The reason is simple: prospective employers have a tendency to request your Github userid and then review your contributions activity to get a sense of your involvement in the dev space. It's not easy to see this type of data via the Bitbucket public profile page.
I would happily move my various open source to the Atlassian workflow if I could get a graph like this on my profile page. But I'd like to see it taken a step further: it should be possible for the public to see a developer's overall activity (private + public) in a graph like that. Sure, you'd want to hide nitty gritty details for private projects, but the activity should be visible in a heat map graph...maybe it could be an option (Show private activity in heat map as well as public?).