ALWAYS use one on a touch screen device, especially when it's not a clamshell design.I'm so torn about whether or not to use a screen protector.
I felt they were necessary with the DS, 3DS, and Wii U because those all used the soft plastic resistive screens and seemed a lot more prone to scratches--especially since they were all being prodded by hard plastic styluses.
With the switch using glass, not being prodded by a stylus, and not being slipped into a pocket like a phone, a screen protector seems a lot less important.
Hmm.
Would you rather develop a scratch on an easily replaced $8 piece of plastic/glass, or a $300 device?