If we have multiple edit log directories, and some of them are formatted, but others are not, we format the unformatted ones. However, when we implemented QuorumJournalManager, we did not extend this behavior to it. It makes sense to do this.
One use case is if you want to add a QuorumJournalManager URI (journal://) to an existing NameNode, without reformatting everything. There is currently no easy way to do this, since namenode -format will nuke everything, and there's no other way to format the JournalNodes.