There is no earthly need to touch your desktop monitor. Ever. It's just stupid.
...until you try to touch your desktop monitor as a reflexive sensible action.
Right now I have two monitors and an iPad (on a stand) next to each other. If I use the iPad for a few minutes - touching it as it is obviously and sensibly made for - I'll often turn to the other monitors and reach out to touch a button. Oh, wait, I can't literally touch the "button" on the screen like I can on the screen sitting right next to it - I have to pause, change mental interface paradigm, look for the mouse, grab the mouse, wiggle the mouse to find the cursor on the screen, then point at the button and click. ...and lament that I couldn't do what I wanted, despite all the technology being available to do so at a very reasonable cost.
Sometimes it makes sense to just touch the screen. Just like on a tablet, there's a button (or whatever UI element), and the natural action is to touch it.
I've been waiting a long time to replace my entire literal physical desktop with a giant touchscreen. Touch is a natural interface. Big screen size is naturally useful (at least for a lot of applications, if you don't have to move the screen). Put 'em together already.
Relevant read: http://www.cooper.com/journal/2012/08/the-best-interface-is-no-interface.html/
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Then the question is, "Who's willing to buy an ipad that weighs 30 pounds?
iMac is just one ounce (or whatever the touchscreen hardware weighs) from being exactly that.
And the answer is: me. If it costs & weighs next to nothing to add the touch capability, then heck yeah do so already. Sure, trackpad & keyboard are the preferable interface for most applications, but sometimes just literally touching the app's UI elements is sensible and easier.
Is the "shut up and take my money" pic verboten now?