TechwareLabs has an editorial on What OS, Windows 7 or Ubuntu? Why not use both!. Here's a clip from the editorial:
Quote: The ability to dual boot systems is nothing new but very rarely done anymore. You more than likely will never find a good enough reason or use for an average PC user to run two Operating Systems (OS). So then you might ask yourself, "Why am I showing you how to?" The answer is easy, to quote Sir Francis Bacon's much used aphorism "knowledge is power ". I personally wasn't very familiar with Ubuntu as an OS so I wanted to learn more. The cost of a second PC wasn't justifiable, just to put a second OS on so that I could learn it for my own gain. So I went and researched how to install both of them on the same machine.