liowik,
Even if the burner was for free, $5 is the price of a SINGLE layer BD-R. This means 1 TB costs $100 in HDD, but $200 in BD-R. If the data is important, you can RAID 1 HDDs and still have the same cost.
Besides, handling and retrieving data from one or two HDDs is much more practical than from 40 discs.
DVD-Rs are still the cheapest way for consumers to backup their data (in GBs/$), though you can quickly get lost in the piles of discs if you want to handle large quantities of data, so I still prefer HDDs. Just make sure the important stuff is in more than one.
Backing up data in BD is only going to become interesting when media becomes dirty cheap, as has happened to DVDs.