Years old issue at this point, but my need for such functionality hasn't changed in a decade.
I guess this makes GitHub, GitLab and Gitorious better than BitBucket since you can use this functionality through markdown. I'll have to move my projects over to a paid Github account since BitBucket can't provide such simple and important tools for complex project tracking.
It makes sense to have sub-tasks integrated in bitbucket.
I found the need for it when I wanted to create a task that cannot be completed before a previous task, but have to way to tie the two tasks together.
Please reconsider (or is there another solution)?
This would make things a lot cleaner for me.
I think the benefit of moving to jira comes in its many other features, not specifically subtasks, so there would still be plenty of draw to upgrading if that's a concern.