Hmmm...not sure I buy that assertion. I understand that you should not drive around in high gear except when driving at higher speeds due to excess wear on the clutch, even though it seems smooth and quieter driving around in high. FWIW, our is driven only on the street. Original clutch still operates as new at 4500 km.
As for the spline wear being a prophylactic measure, sounds reasonable, but it seems to me that as it wears out it may put greater impact loading on the rear end gears. I noticed ours getting "lurchy" before it failed. When you pull it apart and nothing but rust crumbs fall out it and the splines are worn to nubs makes you believe its not intended to fail that way. In my case it was not an instantaneous failure of splines from over-stressing the drive train. I attribute my spline failure (and I suspect some others may be as well) as a direct result of using a lower quality alloy for the shaft or not properly heat treating the splines. I like my Hisun overall, but it does suffer from some design/manufacturing defects. Relative to it's overall value I have found it occasionally irritating but generally acceptable. Sorry if that sounds argumentative...really just sharing my experience with the issue. Regardless, if there's a takeaway from this it's that it is a good idea to pull the shaft and inspect it before it fails, especially if you notice your ride getting lurchy. It's pretty easy to get at.