I ride my motorcycle on the highway with ear plugs all the time. By lowering the wind rushing sound it actually lets me hear the thrum of semi tires (or that jacked up F350 on dirt knobbies) beside me a lot more clearly than if I'm having to try to pick it out through the wind noise.

Using ear plugs can't be compared to stuffing noise making iPod plugs in at all. One lowers the noise level so your ears and brain can better filter for the stuff that you need to hear and the other generates blocking noise all its own in addition to letting the wind noise add to the cacophany.