While ear plugs may not be able to save you from a life threatening injury, they can sure go a long way to helping you maintain an enjoyable life. If you ride on the street at speeds greater than 40mph, then you should be wearing ear plugs. The noise inside most helmets at 60mph is greater than 100db, if you are like me and spend most of your time over 100mph, then the noise in your helmet can be up near 115db. That's louder than the straight through glass pack on your crotch rocket or the drag pipes on your hog. Most of you probably don't even notice. Next time you are out, actually pay attention to the wind noise. Quite loud isn't it? Over time, your hearing will be damaged. How much time? Measurable damage can occur after 15 mins at 115db and after 60 mins at 100db.
Many people claim that ear plugs are a safety hazard. I've been wearing them for years, and I've been able to hear everything on the road that I've needed to hear. I ride 40 miles round trip 5 days a week and usually at least one day on the weekend. Not once can I say that the ear plugs have put me in any danger. If anything, they keep me from being fatigued and keep me more alert.
It's your choice. Personally, I like being able to hear when I'm off the bike.