My Kindle 2 is in an mEdge Go case and it's fine after 18+ months. It's not yellow, the screen isn't scratched (I can't imagine reading with a film over the screen), there's no dirt build up.
I would not call the Kindle delicate, but yes, some kind of a case is a smart decision. I would not just toss it uncovered into a bag or backpack and expect the screen not to scratch. If you prefer to read with it "naked," then get a slip case to store it while you aren't using it. The mEdge Go case is a jacket that stays on all the time, which is what I personally prefer. The cover folds back easily when I'm reading. A "stick on skin" is for aesthetics -- it won't protect the screen while it's in a bag, and I wouldn't rely on it as the only protection.
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