Allow forking and pull requests between Bitbucket and Stash.
Company using Stash 'behind the firewall' can open source their chosen projects on Bitbucket.
Easy backup of hosted Bitbucket repository on premises (if mirroring functionality developed)
Multiple teams can use different infrastructure. Eg. 'core' internal developers using Stash, outsourced guys on Bitbucket.
Bitbucket will have the edge over Github!