I don't know if I'm just "lucky," but I've had 4-5 flats within a two-year span a few years ago. Not off-roading or anything... just normal commuting on normal suburban streets. Just bad luck, I guess.
I don't know how "rare" a flat is these days, but for me they're not rare at all. And I'm glad my car (the one previous to the one I own now) had a spare tire.