It would be advantageous to be able to assign each test to a group of users. The test manager may require a test be done by one of several users. It doesn't matter which user from a group performs the test and any member of that group may perform the test. This would allow a project to have many testers who can work as a team, each performing some of the tests assigned to their group and would reduce the burden on the test manager to allocate each test to a specific user on each run.
This feature may also reduce the risk of an administrator creating generic users and allowing each person to log in under the same account which seems to be the only way to provide this feature currently. Such an approach loses tracking of who did what and I believe contradicts (or at least challenges) the software license (and we wouldn't want to pay you less than you deserve 
).