Yes, it is definitely worth registering if an event has sold out! Once you've completed the registration, you'll be offered a place on the wait list - which means you'll be notified if any spaces become available.
Although there is no guarantee that spaces will become available, there are a number of circumstances in which this can happen. For example, dancers who have already bought tickets can cancel, or an organiser might increase the number of spaces available at an event.
In either scenario, DanceCloud notifies people on the wait list that spaces have become available. So joining the wait list increases your chances of getting one of those precious spaces at the event.