Lets have an instance where all slaves are provisioned via JClouds plugin. Therefore, all slaves has mode set to EXCLUSIVE. When there is a job without label specified (or matrix configuration without label/labelexpr axis) in the queue it is considered not applicable and therefore never executed.
It make sense to me to schedule such a build on slaves waiting for retention time to expire or provision any slave if there is none.
UPDATE: Actually, it does provision new slave for freestyle build without label but it is never assigned.