I strongly concur with @chrillek’s comment here. I always try to ensure that my databases are closed when backing up to Arq, Time Machine or CCC. However, if I do, on occasion, overlook that, I don’t worry too much because whatever else I’m doing I’m certainly not working on any of the databases at the time any of those backups runs.
Edit: I certainly wouldn’t bother to create a DEVONthink archive simply for the purpose of backing up off-site (or wherever else you may choose).
Stephen