I don't reckon it's the nut, I reckon its the tolerance between the shaft and sprocket.
I believe that if the friction between the sprocket and the nut - via the washer, is greater than that between the nut and the shaft the nut will move on the shaft and wear out the threads (mainly on the nut, because its not been hardened). So if the sprocket moves fractionally, the nut will move fractionally, and hence wear its threads out.
And all that said having spent 67,800 miles on 2001 Fazer without experiencing any problems at all, I think that I may well try and demonstrate a solution, to which for me; at least has not been a problem.