DVD's and pretty much all optical media are terrible for backup. they degrade just siting on the shelf. and one scratch and 'poof' goes your data. I'm surprised as a medium that they have lasted this long.
[1]"Manufacturers claim that CD-R and DVD-R discs have a shelf life of 5 to 10 years before recording," note that is "before recording"
[1] http://www.clir.org/pubs/reports/pub121/sec4.html