I have a hard time envisioning why anyone would have serious speed holding concerns below surfing speeds (9mph?) unless youre in a no-wake zone or pulling a small child like ETS mentioned. Either case would seemingly benefit more from keeping a close eye on your wake/child to make sure speed is appropriate, rather than keeping an eye on your speedo.
Skiing speeds are most important for accuracy, but knowing the difference between 47mph and 49mph are more important to me than <6mph. I have some boats that will peg the speedos and then some... so maybe the opinion of this mouth breather doesnt count?