Direct assignment of rights can affect usability.
Directly assigned authorizations and the commands and authorizations in a user's rights profile must be interpreted by a profile shell to be effective. By default, users are not assigned a profile shell. Therefore, users must remember to open a profile shell and execute the commands in that shell.
Singly assigning authorizations is not scalable. Also, directly assigned authorizations might not be sufficient to perform a task. The task might require privileged commands.
Rights profiles are designed to bundle authorizations and privileged commands together. They also scale well to groups of users.