One of the benefits of deploying read-only domain controllers is their ability to recover quickly from an information security breach.
Since only the passwords for a subset of users is cached on the read-only domain controller when these users signed on through the read-only domain controller and the passwords for really sensitive accounts weren't allowed to be cached on the read-only domain controller, the impact of a stolen read-only domain controller is small, compared to a fully-writable domain controller.