Super-linear would make it more difficult for bot owners to estimate how much their vote is worth. But it also increases the value of "concentrated votes" and this is a much more powerful benefit for vote bots, which can easily make big votes due to their high SP. This benefit can quickly dominate over the relatively small issue of trying to exactly calculate the value of their product.
Now, it's possible that some sufficiently lower superlinear amount might not be such a huge benefit, but in such case it's also not going to affect calculation of the value of the vote much either.
I see your point, 50/50 curation would certainly be better, OK, if linear, then flagging will be quite important to do. Otherwise one can always vote alts instead of others and get more curation right?
There's no inherent reward advantage for voting for alts vs one's own account, in either the current rules or the proposed changes.
The only real reason nowadays to vote using alts versus one's own account under any of these rule sets is for "appearances". For example, by voting with alt accounts, you might fool other people into thinking your post had popular support (and conceivably lessen their desire to downvote the post for fear of "going against the crowd").