What is your definition of necessary?
Will it function? of course
Will it have all the same big features like mission control, launchpad, etc? You betcha!
Will it lose any functionality at all compared to the trackpad?* Well I believe you will lose the pinch to zoom and double-tap to zoom (although some of that may be on the magic mouse)
What about the experience/ease of use? Well this is the part that I am questioning (and why I started this thread). You lose the quick and easy gestures for mission control, launchpad, desktop view, pinch to zoom, etc.
*I just realized if you don't have the trackpad or the magic mouse it might be difficult to switch between full screen apps... There may be a keyboard shortcut. I don't have Lion so I'm just going off of what I've heard