Think it's more dangerous to ride without, due to the fatigue after prolonged exposure to noise. A tired rider is a poor rider.
Can't remember the source now, but there's a test in a magazine somewhere that proved that there is _no_ helmet available that provides sufficient noise protection in itself at 40mph pluss. Ear plugs relieves the rider of the discomfort of the constant noise. It also protects the ear from the long term damage associated with that exposure! This is no little concern, hearing damages are very serious illnesses. The numbers of people totally disabled by hearing damage is huge.
I do a bit of sound engineering, using a sound pressure level meter to keep things in the safe area. Can usually guess the dB level fairly accurate now. Got a good idea of what sort of pressure 90 deciBels are, the safe level limit for 8 hours of exposure to sound. .. and 95db for 4 hours, 100db for 2 hours, and so on. An unsilenced motorcycle is about 110dB(according to the litterature, haven't tried!), which is damaging after 30 minutes of daily exposure!
Even with plugs, those 90 dB's can easily be pushed on the motorbike. Particullarly if the speed is high, headwind is strong or the helmet/head gaps makes a lot of turbulence. This varies a lot with the helmet/rider fit.
The most prominent noise comes from the gaps between the neck and the helmet, probably especially so for those gaps making a tube of air going up into the helmet. Try covering different spots on the base of the helmet with one of your hands while riding. The noise will almost disappear when you find the points of turbulence.
Had some success patching these spots with duct tape.
Would probably work better with a flexible material like neoprene. There's neck warmer tubes available that totally fills the gap around the bottom liner of the helmet. Anyone got any experiences with these regarding noise?
Just have to say this as an audio geek: Take care of your ears! All hearing damage is accumulative and nonreversible.