When the time comes, and someone screws up, you will seem like a god when you deliver your backup, whether it's a 3-month-old one-off, or from your crappy daily backup system.
Excellent point! Any tips on how to avoid that, other than not taking the data home / copying to personal Dropbox-type things?
I would document the backup process and communicate it to my manager/client with a mail like "hey, I set up backups, they are stored at <server>, docs are in the wiki".
Other potential issues: causing unauthorized costs ("who stored 10TB on S3?") or privacy violations, e.g. when working with healtcare or payment data.
If it stored credit card data or other important stuff I'd take a look at what PCI compliance says you have to do for your backups and follow that.
Even if it's old, when you're facing no data or old data, old data looks like heaven.