I've been a JIRA and Bugzilla admin in past jobs, and have quite often had users ask for the ability to have more than one assignee per issue.
I know this is possible in JIRA, but to my mind it never makes sense; an issue should represent a piece of work, and only one person can do a piece of work (at least in software, I've never used an issue tracker for a 2-man bobsled team ;-)) A large piece of work will obviously involve more than one person, but I think in that case it should be split into subtasks to allow for accurate status reporting.
Does anyone have any use cases where it's valid to have multiple assignees ?
The Assignee field means many things to many people. A better name might be "Responsible User". There are three cases I discuss with my clients:
A. number of assignees = 0 JIRA has an Allow Unassigned issues option but I discourage use of that because if a work item isn't owned by anyone it tends to be ignored by everyone.
B. number of assignees = 1 The default case
C. number of assignees > 1 Who is responsible for the work item represented by the issue? The best case I've seen for this is that when an issue can be handled by any one person in a team, so before triage the issue is assigned to everyone in that team. I think a better approach is to create a JIRA user with an email address that sends to the whole team, and assign it to that user. Then a member of the team can have the issue assigned to them in particular.
Changing the one assignee case has the history recorded in the History tab. Nothing is lost in that case.