Has anyone encountered this kind of policy before?
And does anyone know what the security benefits are supposed to be?
The security benefit is that the devices exposure is greatly reduced with little cost in terms of availability.
In security the risk to a given asset is described by three factors: threats, vulnerabilities and exposure.
The Blackberry is exposed to a wider threat environment whenever it is remotely accessible. That is whenever the network capabilities of the device are active; a threat (person) could attempt to remotely gain unauthorized access to the device. If the network capabilities of the device are inactive then a thread (person) must be physically adjacent to the device in order to gain access. Generally users do not like to lose network access, but when the user is charging the device, they are typically not using it. Most mobile devices get charged over night when their users are sleeping.
Additionally, if a threat was to attempt to access the device remotely and the user was interacting with the device they may notice some odd behavior. If the user is not interacting with the device they would not be able to observe any anomalies due to a remote threat.