First of all, he can still burn DVDs without iDVD, there just isn't a standalone app for it anymore. Leo doesn't really think backing up to DVDs is the best way to go. They decay imperceptibly, so he would have to check them regularly to make sure they're still good. Leo says backing up to an external drive is much faster and better, and then also to the cloud using a service such as Carbonite. That way he has three copies and in two different locations.
Leo does think physical media is going away in favor of the cloud. But the only Macs that don't have optical drives right now are the ultra-thin ones like the Airs and the new Macbook Pro with Retina display.
(Disclaimer: Carbonite is a sponsor).