Correct. My bike has a stock exhaust and sounds like a sewing machine, but I wear earplugs mostly to protect against the constant wind noise at speed.

Is that not accomplished by wearing a helmet? I haven't been on a motorcycle in over a decade, but I recall it being pretty quiet in the helmet.

No, there's more than enough noise to cause permanent hearing damage.

Obviously higher speeds=higher wind noise; you might be fine < 35mph (~ 55kmh) in a city, but anything above that hearing protection is advisable.

"Measurements of motorcycle riding noise levels vary, but are generally around 85-95 dB at speeds up to 35 mph, climbing to 110-116 dB at 65 mph"

I had a pretty nice helmet, and it was absolutely not quieter