I use both Bitbucket and GitHub. I specify this in my package.json file as follows:
This should be valid. When I publish this there is no "github" link on the npmjs page. Ideally there would be links to both Bitbucket and GitHub.
Pick one, either github or bitbucket. If you submit a pull req that auto-links bitbucket-hosted projects to their appropriate bug tracker and doc pages, I'll accept it happily. However, since 99.9% of node programs use github, it seemed silly to spend time on.
Regarding the spec, CommonJS is dead. It served its purpose well, and lives on in our loving memory.
Would you be entirely opposed to a pull request which allows use of the repositories key in package.json? Git, by its distributed nature and overall design, allows for and even somewhat encourages the use of multiple origins. After all, it's safer that way™.
By all means HAVE many repositiories. If one goes down, then tell the npm registry where the new one is. But there's no benefit to having multiple entries in the package.json file, and many drawbacks.