Re: Using a pencil is great
Exactly. The same problem is rife in anything that you can "just draw on".
I work in schools. I've seen many Smartboards etc. ruined by "this marker pen I found" or even ballpoint pens on soft, squidgy, touch surfaces. And even the kids in rougher schools realise that a well-place ruler throw can knock the lesson for six if they can stop the board working.
The alternative is fancy-schmany pens that cost £60 each, on a solid surface, and you can't use any substitute, or even IR-detectors and anything shiny on the board - and both are just worse options.
Although, the last Hitachi interactive whiteboards I bought actually came with 5 layers of peelable surface for just such instances and you could buy more. But we installed them, never told anyone (because I just know they'd peel them off for the slightest smudge), and then everyone who knew changed schools. I guarantee they were just binned after a while because nobody realised that there are four more layers underneath, ready to be used...
Same school had touchscreen all-in-one PC's with hard-glass touchscreens. Last I heard, the cleaners were smearing them with desk cleaner spray three times a day because of the fingerprints and the streaks that not using a proper cleaner was giving them. The screens are probably like looking through the bottom of a submarine by now.