I've had a laptop with a touchscreen for about two years. I didn't even know it had a touch screen for some time (linux - it wasn't auto-detected at the time, and one day I was wiping a dust off the screen and noticed the pointer had moved there:) )
It now has many input devices; a keyboard, a touchpad, an independent graphic tablet and a touchscreen.
I most of the time have my hands on the keyboard, to type and use keyboard shortcuts, and use the touchpad to point and click on small things (like when I want to select a specific sentence in a webpage). I use the graphic tablet when working with graphic programs, and I use the touchscreen when I want to click or move a large area. For instance to bring a window to front I often find it easier to quickly touch it rather than reaching for the touchpad, looking for the mouse pointer, moving it around and clicking, especially as my fingers are on the keyboard, the screen is no further away than the touchpad! To move a window I sometimes press alt with the left hand and move the window with the back of a finger (alt-click-drag moves a window on most linux window managers). When browsing an image folder touching the image I want to display is much easier than with the mouse, etc.
In practice I touch the screen maybe a couple of times per hour, and I share this experience that at work or at my parents' place (no touch screens there) I regularly find myself wanting a touch screen.
About smudges I use the back of my finger to touch the screen, it works just as well and leaves practically no marks.
*Clicks Preview with a finger*