The idea of diminishing returns would make it less attractive to upvote the same authors (including oneself) again and again. I described it like that:
"How about if after each vote on a specific account (including ones own account) each further vote on the same account would lead to significantly less curation reward for the voter and less profit for the upvoted account? Thus, when upvoting an account which I had already upvoted before, my voting power would be smaller than in case I upvote an account which I didn't upvote before."