You wouldn't just flip your computers power switch to turn it off would you? (I certainly hope not anyway).

The OS for a computer, or console for that matter,could be running any number of applications in the background that you don't even know about. Suddenly cutting the power could lead to things like corrupt or damaged files, and other problems. Maybe this never affects you, maybe you turn it on one day and find it can't load the XMB. Also, it's generally a good idea to not expose your computer or console to sudden power surges like that as it could possibly damage them. Again, maybe you never have a problem, or maybe they crap out on you in a year. The point is, unless you like wasting time and money, play it safe and shut them down properly.