Is it really so hard to imagine a task for which gesturing is appropriate?

Yes. Can you think of some examples? I can't think of a single task I'd like to do on a desktop computer that wouldn't be easier and more-precise with a mouse. You know what's easier than pinch-zoom? A scroll-wheel. Multi-touch is great for phones/tablets, but this seems pretty silly to me as an interface for, y'know, computing.

Currently we have mostly just mouse+keyboard, so it's pretty much expected that majority of present applications would be optimized to work best with these inputs.

Once we will have more expressive inputs, new types of applications will start to pop up.

For example, I often work with 3d. For such applications, mouse + keyboard controls feel very clunky. Even simple tasks like setting up cameras or placing objects in the scene are pain.

It would be much more natural to manipulate 3d objects using gestures in 3d space.

Let's say I'm coding on the laptop, and I want to scroll the docs that are up on the monitor. Three things need to happen here: 1) focus on the docs, 2) scroll, and 3) focus back on my work. My goal of course is to do this as swiftly and effortlessly as possible.

So, using the keyboard: I alt-tab from editor to browser, hit the space bar to scroll down, alt-tab back.

Alt-tabbing is more of a hassle the more programs I have open. Additionally, the scrolling is either a lot (page down) or a little (down arrow); usually what I want is something in between. It could be made easier if I had a specific keystroke set up to switch to Chrome, or to scroll by x amount, but that's far from intuitive.

Using the mouse: I move my hand down to the trackpad, or over to the mouse. Wiggle it around briefly to locate the cursor, then drag it across the screen to the second monitor. Use two-finger scrolling or the mouse wheel to scroll to where I want to. On OS X I blessedly don't have to click to focus scrolling, so I could start typing immediately, but to avoid confusion (and because I want to scroll my code) I need to drag the pointer back.

This provides a much better scrolling experience, but the process of moving focus across monitors with a pointing device is a huge drag.

Using a hypothetical desktop gesture reader: I lift my hand in front of the second monitor. This focuses on the window. I draw two fingers down (or up) to scroll. I drop my hand back to the keyboard and begin typing immediately.

My elbow stays on the table, so it's quite comfortable. I have excellent granularity in scrolling— not as good as the mouse wheel, since I'm not actually touching the screen, but probably about as good as two finger scrolling. Most helpfully, the task of focusing on the browser and back is about as effortless and intuitive as it possibly could be without reading my mind.

Coming back and using a desktop PC with a mouse felt very strange - so I'm now far more willing to consider that approaches like this might be feasible after all.