It's a valid use of GPL in dual licensing context. Other examples of dual licensing are: VirtualBox, MongoDB, BerkeleyDB. All patch contributors who initially

It's a valid use of GPL in dual licensing context. Other examples of dual licensing are: VirtualBox, MongoDB, BerkeleyDB. All patch contributors who initially chose GPL side of dual licensing model should sign copyright disclaimer for their patches, otherwise their code won't be accepted into the main repository. Alternatively, patchmaker may execute his/her GPL rights and fork the project (e.g. Percona).