With our previous system (granted, mostly due to it's instability), we had incremental backups running surprisingly frequently (every 30 minutes). It sounds a bit mad, but it worked well to keep our window of uncovered time very small, and since incrementals only copied transaction logs, it did not cause any noticeable load on the server. Would there be any unforseen gotchas doing the same thing, maybe a bit less frequently, with Zimbra?