I believe a trackpad and a mouse are just different and each have different strengths. My opinon, in general, is that the trackpad is more "natural" to use. I think it shines on scrolling (like in web browsers or scrolling documents) and on gestures which can be important on the Mac (switch desktops, mission control, etc). But the mouse is stronger when you need to move the pointer quickly (like a game), need more precise pointer control (selecting text, for example) or when you need to click-and-drag. Click-and-drag is a big problem on a trackpad. So I have both in my desktop setup since each are useful.