One of the worst mistakes you can do on a computer is not have a back-up browser.
Suppose a file on your hard-drive that makes your browser work gets infected or unreadable. What are you going to do? You won't be able to access the internet to re-download it or another browser. You would basically be stuck.
It would be real easy to click another icon (Opera) on your desktop to get you up and running again.
Also last month IE7 had a Security Warning and was recommended not to use it until they fixed the issue. They had it fixed the next day, but having another browser on your computer made it easy to use something else while they fixed the problem.