Requesters/End-Users leave organizations all the time. If someone has many tickets open, it's time consuming to go through each one and change each one manually.
The standard Bulk-Edit dialog should be updated to include requester in the list of fields.
It's been quoted as a "security issue" before, so perhaps it should be Admin only if that's such a concern.
The API is not an easy solution for Agents who are responsible for managing their Organization's tickets.