I'm just guessing here, but I'd be willing to bet that 99% of riders of Harley touring models would never adjust rebound damping. On the other hand, serious sport bike riders probably wear out the adjustments searching for the "sweet spot".
The nonadjustable shocks on the market have compression and rebound damping rates set for average conditions, and are in some ways like "one size fits all" clothing. It works for many, but not for all. If you are far from average, in terms of riding style or amount of weight you carry, perhaps having an adjustable rebound setting would be beneficial. However, that doesn't mean constantly screwing around with the setting, it just means taking the time to fine tune the damping and then leaving it alone. Having adjustments doesn't mean you aren't getting your money's worth if you aren't constantly adjusting something.
Jerry