Ear plugs are more for wind noise at high speeds than loud pipes but, in any case, it is a good idea to wear them (if legal in your state). I always wear them for longer trips on highways (I usually cruise around 85mph)
Not wearing ear plugs on long trips will guarantee some hearing loss long term...lots of studies on that.. The extent of hearing loss is cumulative and based on decibel levels (higher db, less time to cause permanent damage and damage can creep in as little as 15mns at high db levels. You may not notice until you decide to take that hearing test later on...
I am actually more aware of what is going on around me with ear plugs on since I can ride hundreds of miles without getting tired and I can still hear horns, sirens...and fellow harley riders...