I use touch screens as part of my job on some of our control systems. The commercial systems don't seem to suffer too badly from extended periods of not-very-careful use by the users, even those who think it's a good idea to use the pointy end of a biro....
However, they are not anywhere near as comfortable to use as a traditional keyboard and mouse. Problems I've come across:
1) It's easier for someone behind you to watch you enter sensitive information, if the screen displays a keyboard.
2) There's the danger (has happened to me many times) when showing someone else something on the screen that you touch a button/icon that you simply just meant to point at!
3) Entering data in a speedy fashion is more difficult, especially if the screen is near vertical.
4) If the calibration of the screen isn't spot-on it can be a frustrating experience!
5) If you wear bifocals/varifocals, typing anything using an on-screen keyboard is very uncomfortable indeed. You'd be surprised how quickly it makes you irritable....
Anyway, that's my tuppence worth.