There is no advantaged to the IS backup, it is a REQUIREMENT!
If in a worse case scenario your database is hosed, you cannot recover Exchange using mailbox backups. Only the IS backup can get you up and running (or a disaster receovery)
Mailbox backups are just for that, mailboxes. If you need to restore individual or groups of mailboxes, that is the fast way to do it. If you did not backup mailboxes seperately, then you would have to restore the entire IS which would take a long time compared to just a few specific mailboxes.
The IS backup is the database itself, which also includes all the individual mailboxes. The difference here is it is not backed up in a way that you could restore individual mailboxes from. Restoring the IS would restore all mailboxes as well.
If I was you, I would run at least one full IS backup each week, suplemented with differential and incremental backups throughout the week. If you run Full mailbox backups everyday, use wildcards to exclude the Sent Items and Deleted Items folders from every mailbox to make the backups smaller. If you only do one Full mailbox backup, be sure to use differentials as well since it would make your overall backup window much smaller everyday for mailbox jobs since they tend to be quite slow due to Outlook/Mapi reasons.
Hope this helps.
