OP asked about low revs on the Highway - assume he means cruising.
Pretty simple, vibration and engine noise.
Generally speaking the higher the revs the more vibration and engine noise regardless of single, twin, triple or 4 cylinder. All engines have a sweet spot for power and curising (where vibration and noise are not as pronounced) and often the two aren't in the same rpm spot.
If one is crusing at 75-80 mph, you don't need much more roll on power power too initiate a pass since you already running 80, so why not raise a gear to lower rpm as long as there is enough Tq to kep you moving at the same speed without engine lugging.
If I need to quickly pass at 80, or any other speed for that matter, I can drop a gear as quickly as I can wrap the throttle, and that is exactly what I do.
Cruising on the highway I'm going be in low revs, riding the twisties or in heavy traffic where aggressive manuevers are needed, I keep the revs up.
Any day on a dirt road is a great day