You make using a mouse sound an order of magnitude harder than it actually is. It's not a nightmare at all, and plus, your arm is resting on the desk like with what you're already used to. Think about eating dinner, or writing a note.

In addition to that, using your finger is not nearly as accurate as that "tiny little arrow" (which is of adequate size), and it obstructs your view of what you're pointing at. Touch screens are only good for big, bulky, simple UI.