With a wide range various things could happens to your computer, it is best to store your bitcoin on an exchanger such as mtgox and campbx. Your computer could crashed, hard drive crashed, hacked, and so on. Even if you have backup, it’s a waste of time restoring your computer restoring your Bitcoin and what if your back up also crashed, or back up CD defective not readable or lost?
Based on historical lost of bitcoins from mtgox, the customer’s account of bitcoin was not compromised, even though if it was compromised I believe mtgox will take the responsibility of those lost bitcoins. I believe the same with campbx although I don’t recall any news in the past where their customer’s account bitcoin was hack.
So it’s important to store your Bitcoin at a safe place where you can cash them quickly. So far, Mtgox and Campbx. However off course there will always be a risk everywhere, if these two exchanger get shut down by government then say good bye to your bitcoin, but at the same time Bitcoin era would be over as well, so don’t worry 🙂