Question: Are there any guidelines as to when it's useful to have a touchscreen interface? My first experience was with my smartphone and I thought the touchscreen was great and a huge improvement on usability. I then bought a tablet and I find the touchscreen isn't as useful. This is because tablets have larger screen and it feels like I have to do much more work to get things done, for example on Androids just pulling down the menu bar from the top is a fairly large motion on 10 inch screen where as on phones doing the motion is easy. It may have been better for large screens if the user just had to tap something to get it to move, rather than drag their finger 10 inches.

Also what is the point of multi touch screens? The only thing I can think of is zooming in and out of photos. Even then it's not that useful because there could be a button for zooming in and out.

Answer: 
I don't know of any studies, but I do know that I was surprised to discover that I do use the touch screen on my windows 8 ultrabook a lot, frequently in desktop mode. I especially like to scroll by swiping my finger on the touch screen rather than by using two fingers on the track pad. Also it is often faster to just reach up and tap an ok or save button than it is to drag the cursor to the correct place and then click. Obviously if you have a mouse attached to the laptop that is faster than a track pad but not necessarily faster than the touch screen. 

I would not want to interact with my laptop only through the touch screen, though. It is more like a useful additional feature that speeds up UI navigation as opposed to a stand alone way to navigate the UI. This is because not all the actions you can take in the OS have been assigned an efficient way of being taken through touch. 

I also have an 8" windows tablet. I almost never use a mouse to navigate it - instead I use a pressure sensitive stylus, which is somewhat different than touch but is a similar premise. Handwriting recognition is excellent in Windows 8 so I can easily take notes and can edit documents to some extent although the mouse is better. I downloaded a program called Touch Me which allows me to assign actions to various gestures and to customize the touch screen experience. 

I hear what you are saying about the size of the motions required to navigate larger screens although you usually don't have to swipe all the way down to pull down notifications... A few inches suffices. If you root your tablet, you can install an app to assign actions to gestures and you can design how much of a swipe/gesture is required, and the gesture will perform that action from everywhere (ie not just on the home screen). You can program some gestures without rooting on android, but they won't work everywhere. For example buzz launcher's gestures only work on the home screen.
I am not aware of any studies trying to argue touchscreens aren't useful and in any case it's either an addition or the only option. On laptops you can still use your mouse, and on tablets using a trackball or anything similar works terribly and mouses/keyboards are impossible.

> 
> Also what is the point of multi touch screens? The only thing I can
>  think of is zooming in and out of photos. Even then it's not that
>  useful because there could be a button for zooming in and out.
> 
> 
> 

A button just expresses the wish to zoom in or out, the multi touch gestures express the wish to zoom in and out, where to zoom in and out and by how much to zoom in and out. That's quite a bit more information. Aside of that I would argue they are 'only' useful for games and gesture shortcuts (things you can do through menu's as well, but are faster with for example a two finger swipe from the left).