Why do you need to backup your stuff? You can always create more stuff, right? It doesn't work that way in real life, and it doesn't work that way for your data.
Who uses more Hard Drives? You, me, or Google? We can study the impact of a single failed Hard Drive in your computer, but that doesn't make you an evangelist for data backup unless you have lost data. Google has more Hard Drives than you can imagine. They know what is happening with data, and they closely track the health of their systems.
They completed a study in August 2007 that tracked a 5-year history of Hard Drive failure.
1 Year ~ 7%
2 Years ~ 8%
3 Years ~ 8%
4 Years ~ 6%
5 Years ~ 7%
This shows that your Hard Drive has around a 25% chance of a serious error in a three year span.
Why are you not backing up your data?