I wear earplugs whenever I am out on my motorcycle. At speeds above 35mph or so, steady wind noise in and around a helmet gets deafening... for real, deafening, as in demonstrable permanent hearing loss.

On bikes, it's not so bad. I think if my average speed ever gets up to 20mph or so, it might push me to wear earplugs, but for my commute, with no huge downhills, no earplugs works for me.

I agree that all sensory input one can get while mixing it up with traffic is important, so I'm hesitant to use earplugs while riding a bicycle.

Although earplugs do come in varying degrees of sound reduction--look for a lower number for bicycling perhaps. For motorcycling, I find the highest number I can find.