First, I think Multi-Touch is being used as a buzzword for the overall iPhone GUI ... which is not just Multi-Touch, and doesn't make full use of Multi-Touch. It's more accurately described as a finger GUI (as opposed to a pointer* driven GUI, or a stylus driven GUI).

(* the thing on your screen is not a mouse, the mouse is the thing next to your keyboard ... unless you have a trackpad or trackpoint ... either way, the thing on your screen is a pointer, not a mouse)

When is multi-touch useful? Any place where it makes the gestures of the GUI more natural than a single-touch finger GUI. For example, pinching to zoom-out and unpinching to zoom-in are both more natural and more granular than a clunky pan and click form of zoom-in/zoom-out. And the current NIT implementation of that is worse than, say, the google maps version. (I mean, really, zoom-in and zoom-out buttons on the side of the device? what is this, 1983?)

When is the iPhone finger GUI useful/important? Same as above: any place where its gestures are more natural than, say, a stylus or button driven interface. For example, scrolling via a scroll bar is great for a pointer driven interface, but it is not as natural as literally grabbing a list element with your finger and moving the list around. In that case, the only thing you need a scroll bar for is to help you understand where in the larger list you are.

So, while a big ol finger-fat scroll bar is better for a finger driven GUI than a narrow stylus-wide scroll bar, it's actually a stupid way to do a finger driven GUI. It's trying to scale the stylus GUI up to finger size (where a stylus GUI is typically already a kludge to try to make a pointer GUI work in an environment that isn't suited for a trackpad, trackpoint, or mouse), instead of trying to figure out what widgets and behaviors make sense for a finger driven GUI.

What the iPhone gets right is: designing the interface around the input mechanism (the finger), instead of trying to adjust a completely different interface, designed for a completely different input mechanism (a mouse) around accommodating your finger.

Multi-Touch is one piece of that finger GUI. It's a useful part of that finger GUI.

There are many things that Steve Jobs says that are just posturing, but one of the things he was right about last year was that it's backward to require a stylus for a PDA. I think a stylus has its place (precision drawing, and such), but it's not as the general input device for a touch-screen interface. The general input for a touch screen interface should be your fingers. And Multi-Touch lets you increase your vocabulary of gestures by using multiple fingers at the same time.