If you don't believe a plastic scraper can damage your base just as easily try this:
Take a sharp corner of a plastic scraper and dig that into a nice smooth ski. Not pretty. If you had done it on a window, there would be no damage and the scraper corner would be dulled. Unfortunately, our skis are softer than glass.
The problem with metal scrapers is that they are ductile, and can easily have a burr. The 600 grit sandpaper will remove that, but even then, when waxing racing skis which will actually see a race, I take the time to use a plastic scraper which is sharp enough to remove the wax without bearing down so hard it flattens the structure. For big fat powder skis I use my smoothed metal scraper---thin enough to go through a thick pile of wax, never needs sharpening.