I have the magic mouse for when I need to do CAD work - although I don't use the external mouse nearly as much as I did with a Windows computer.
When I sit at my desk I have Synergy running so that may count as an external mouse and keyboard i.e. - I use a logitech keyboard and mouse connected to a windows computer to control a windows laptop and my macbook pro. Synergy
I agree with the above in that it depends on your needs and preference. If you are one of those people who uses their laptops like a desktop then I can see how a person would want an external keyboard, mouse, and monitor or have the laptop on a stand. Also ergonomics is another reason but I have my laptop setup so that it's ergonomic for me to just use the built in keyboard and screen.
For me my laptop isn't always used at my desk so it would be kind of a waste of money for me. The only external device I have to have is my Logitech mouse. There are many times I have to use precision fine mouse movements for work and other computer projects so the trackpad isn't good enough for me. Also an external mouse is small and portable so it's very easy to take along with me unlike an external keyboard or monitor.
I also like going back and forth between my mouse and trackpad for the gestures but it also helps reduce my hand from ergonomic pains because I'm not using either one for that long. Going back and forth reduces the repetitive muscle stress.
Since I use my MBP as my central media center, I need to have more control over it. I have it hooked up to an external monitor and put the computer in clamshell mode. Then I can control it with my bluetooth Mighty Mouse and wireless Apple Keyboard.
I think the largest factor is whether you plan on using an external monitor with it. If you do, then yes you will probably want the externals. If, however, you stick with the laptop monitor as your primary/only display, then i think its counterproductive.
Up until now, I have been using my MBP keyboard, but mainly due to lack of space, but we are moving into a bigger apartment in a couple of weeks, and have already ordered a large office desk, where I plan on placing my MBP on a stand, buying an external LED 27" screen, and then going for the Apple Wireless Keyboard, and Magic Mouse route.