I don't know about that. I mean, nature does love the bell curve, so I think that 3d6 is definitely more realistic, but for a game I think that having a 1 in 20 chance of getting the best outcome (as opposed to 1 in 216) is more dramatic and more fun.
Also, +X/-X items and any conditions or buff/debuff effects that hand out flat bonuses or penalties do really strange things to probability on 3d6 (if you're using them like a D&D 3.x or 4e d20).
