My suspicion is that anyone buying an N900 that has been used for six months that hasn't been used with a screen protector will find at least some tiny scratches, like this one has, so I agree.

I was surprised too when I saw a couple of light scratches due to stylus use. That was just over a period of 2-3 days until the Zagg Shield arrived.
If scratches are visible, adding a screen protector makes them go away, at least in my experience they're not visible anymore.