Bugzilla has a feature called "Users Watching". When you enter one or more comma-delineated user accounts (usually email addresses) into the text entry box, you will receive a copy of all the bugmail those users are sent (security settings permitting). This powerful functionality enables seamless transitions as developers change projects or users go on holiday.
In general, watching allows to share the load for tasks a person is assignee, subscriber or watcher of. Watching the user as a whole allows to achieve the result with a single control, without changing projects or tasks.
- In most cases, a user will receive notifications out of ad hoc subscriptions and task relations rather than one-size-fits-all subscriptions to a project, as well as file tasks outside of their watched projects. Watching allows to easily follow what they're doing or are supposed to read.
- Even assuming the person never adds or remove self to cc, nor is added or removed from cc, nor files or is assigned tasks outside of their watched projects, copying all the projects subscriptions is tedious and impossible to reverse cleanly (a manually-curated list of the reasons a project was followed would be needed).