It's great that this functionality is there and it also works properly, however I've been exporting my calendars manually for ages because I didn't trust MozBackup on it ... quite simply because it wasn't listed in the export options. It might be a good idea to make this clear to the non-expert users :D
Since Lightning is so popular nowadays - and quite well integrated with the latest Thunderbird - a lot of serious users probably also have calendar and task data to backup and import, wouldn't it be worthwhile to program MozBackup to detect if calendars are present and present the user with a checkbox for those as well? This also gives the user control so can disable it. And from that perspective it might be a good idea to also make a checkbox for tasks, so they have separate control over tasks/calendar backup/restore.