I hit upon this question while looking for solutions to doing this. Since I want to do this, I'm guessing my use case counts as an answer to your question: I only really want one assignee in the sense of someone currently working on a problem, but I want to track the whole lifecycle of an issue. For us, that can mean:
- A support person receives a report from a customer, creates an issue
- An issue-wrangler reviews the issue to make sure it's valid, not duplicated, has all appropriate details, etc.
- A developer implements/fixes the issue
- A tester performs whatever tests are appropriate (in our case, mostly extending our automated testsuite to additionally test the feature/fix)
- An operations person rolls out the new version to a test environment
- A support person informs the customer, who does his own tests with the new version in the test environment
- An operations person rolls out the new version to production
Not all issues necessarily go through all steps. Some issues have more steps (e.g. a code review between step 3 and 4). Many issues will also move backwards among the steps (developer needs more information, we go from step 3 to 1 or 2; tester spots a problem, we go from 4 to 3).
At each stage, only one person is actually responsible for whatever's got to be done. Nevertheless, there are a whole bunch of people who are associated with the issue. Tracking systems we've used are happy to offer easy changes to previous owners of the issue (shown as a list), but I'd ideally like to go a step further, with the owner automatically reverting to the correct prior owner depending on the issue's status. At step 6, the original support person from step 1 should ideally contact the customer. At step 7, the ops person from step 5 would ideally be the assignee.
In other words, while I don't want multiple assignees for a given step, I do want there to be a "support assignee", a "developer assignee", a "testing assignee", etc.
We can do this with subtasks and we can do it by manually selecting previous owners when changing statuses, but neither is ideal and I think the situation above is one where multiple assignees would make sense.