Notwithstanding the separate debate about using bid bots. If a post is not worth promoting after 3.5 days then how much does it add to steem/the world? That is one of the issues of the 7 day payout rule. It reduces the incentive for material that lasts.
WRT promotion itself however - my understanding was that votes were intended to allow humans to weed out the good material where bots don't do that. They promote the material of people who can and want to pay for it.
Before the 3.5 days time-limit, abusers would buy upvotes for their post at around 6.5 days which would reduce the time services like steemcleaners would be able to remove rewards in abuse-cases.
Thank you for your generous upvote.
Do you know why steem stopped the 30 day payout they used to have?
I guess the truth is no matter what great thing is added or changed people will find a way to "abuse" it.