It's about tradeoffs. Positives for touch screens include:
Low cost and small physical space requirement, assuming an appropriately sized screen is already part of the design. You can often fit a bigger screen by foregoing physical buttons.
Direct rather than abstract interface is easier to instruct 'press the picture of the item you want' vs 'use the arrow keys to highlight the item you want, then press the button to select it'
Reconfiguring the interface is a software change (and generally inexpensive) vs a hardware change
Negatives include:
Lack of tactile feedback, generally, means more focus is required to confirm desired actions.
Users hands and arms aren't transparent, so it can be difficult to see the screen while interacting with it.
Context can change while interacting which can be confusing (this isn't exclusive to touch screens)
Filling input buffers is harder. On old Nokia phones, you could navigate menus quickly by pushing number keys and get ahead of the UI, but if you knew the sequences, everything would work out. It's a lot harder to do that on a touch screen if you're pressing buttons that change the on screen button layout --- you're going by memory for the screen location without tactile feedback. I think uis are more likely to discard early taps than early keystrokes, making it possibly less reliable as well.
If the touch sensitive layer is widely seperated from the display layer, calibration to converge touch and display becomes sensitive to viewing angle and can be difficult to use. I'm thinking of touchscreen atms and touchscreen soda dispensers that I often have to offset my touches so they register.