I agree with the OP. Having plugs or music seems to calm everything down. I can still hear stuff, just not as loudly. As for me....I bring music on nearly every ride; and I ride A LOT. I have yet to be suprised by anything other than a passing cyclist who has not indicated intention to do so. I still hear EVERY SINGLE CAR coming (Oh, except for a high speed & completely out of control pickup truck that slid by me sideways on ice this winter; but it would have done no good to hear that one anyway). Most tires on the road seem to make enough noise that engine noise is not needed to be aware that they are coming even with plugs or headphones.

With all respect, I think you folks who argue that no-one should wear earplugs while riding have not spent much time riding with plugs or music and are just basing your statements on your own unfounded but strongly held beliefs. My father was a banker and when I used to spout my liberal nonsense to him he would say "never underestimate the conviction and power of the uninformed".