The motor R+R is always big fun at a Bug In.
Spline wear is due to a couple of things. The two issues work independently, and also amplify the other. One issue is the drive disk isn't centered on the disk spline. I don't mean that the clutch splines overhang the gearbox splines, which they do by a small amount but that isn't the problem. But that the clutch disk isn't centered on the clutch hub, it's offset to one side. The other issue is the gearbox shaft is unsupported at the crankshaft end, and supports the weight of the clutch disk without the benefit of a pilot bearing.
The offset-from-the-center drive force causes the clutch spline to deflect radially on the gearbox spline. (Maybe rotates on it's axis is a better visual, like how the Earth is tilted on its axis.) That deflection happens during the shock accelerations of gear changes, when the clutch disk rapidly changes velocity as the gearbox ratios change. The clutch hub momentarily 'tilts on the splines' if you will, and that wears away at the gearbox splines. If the drive force were centered on the spline that would not happen.
The other issue is when the clutch plate is retracted, the unsupported gearbox shaft falls against the weight of the clutch disk. This movement causes a slight misalignment between the clutch disk and pressure plate which 'tilts the splines' just as above. The effect is also the same, it slowly wears out the clutch splines.