If you're gonna have multiple displays, a Magic Mouse, and a keyboard, then you might as well buy an iMac instead of a MBP.
How did you logically come to that conclusion? There are plenty of other considerations when someone chooses a laptop over a desktop. One is usually that they need something portable... Plus having room to spread out on a desk, with a KB, real mouse, and another monitor makes working much more comfortable. I even take a bluetooth mouse with me because using the trackpad on a MBP can feel cramped for long periods of time.