I've had mine for about 7 months and there has begun to appear small 'smudge-like' blemishes on the ASF. They're practically unnoticeable in daily use, partly because they're so small and 'light', but mostly because they're covered up by all the fingerprints. This is how I came to this thread, I noticed the fingerprints and thought perhaps I should test a scratch-protector, decided to wipe the phone to show off my 'protector-free spotless screen', but lo and behold, there are (albeit very slight) imperfections below the smudges!
While considering the period of use (7 months, not long, but not short either), these 'scratches' are hardly noteworthy, but I could definitively image it becoming a "problem" after 2-3 years, though it certainly depends on the care of which you handle the phone. I too keep my phone alone in the pocket as I'm sure keys would make quick work of the seemingly malleable ASF. I've twice during this period dropped the phone behind the bed, but never any significant (read: pavement) or high-height drops. But if you want your phone to stay mint then I'd probably have to recommend a scratch-protector. Perhaps even wiping the phone with certain kind of fabrics create these slight blemishes as well, or at least worsens prior scuff-marks.
But I'm worried about your opinions on the screen-protectors since I too have always disliked them (from afar, I've never actually used them myself.) I used my phone without any sort of protection for the first 6.5 months, just recently I received a semi-thin (terrapin) case because I noticed how bad the plastic backside was getting scratched for some reason -- worst part is I like the terrapin, which I didn't think I would, so I wish I'd gotten around to it earlier. When compared to the plastic backside, the ASF is pure diamond.
Think I'll order some type of scratch-protector to see if I can live with it, but I'm also not too worried about making the screen unreadable by not using one.