Non-linear makes it more difficult to calculate bidding price and voting amount for bidbots. Say, if a bot has 100sp, under linear, if 10 ppl bid the same amount, they all get the 10sp equality of voting amount from that bot, whigh is 1/10 of its full voting amount. Win-win for all.
But under n^2 for example, the voting amount 10sp generates only equals to 1/100 of that from 100sp, which makes bidbots less attractive to use. I mean you can still use it for promo but looks far less attractive as Some bidders are gonna lose money by bidding.
With non-linear voting, we could actually see some bidding for the bidbot votes, as the bidbot would only be giving one 100%-vote every second hour :-)