I am using bitbucket for a couple of years and all that time I was doing some small projects stuff with 2-3 persons.
Bitbucket is getting better and in from my point of view (as a small team member) it could cover all aspects of the project. There are repo, wiki, issues, team management, downloads.
I know that you are trying to keep issues tracker thin and I know that Jira should satisfy all need of advanced project management and project specific stuff. But there are a just very simple things that could make Bitbucket already self contained. I am about some visualisation tools like progress bar to milestone or version. It is not something very complex and it should not be, just small progress bar with configuration what issues are closed for particular milestone or version. At that is all. I believe many people will find that stuff useful.
Before writing that issues I tried to integrate Jira and it was not as perfect as I thought it should be. For example I could not find the way to import issues from Bitbucket to Jira if there were no specific labels in issue names and commits.
Hope there will be some positive feedback or ways to do it with plugins or smth.