You are right about the value in promotional services. It is misusing the reward pool mechanism for that purpose which is the real problem.
We have the promotion tab, but the problem is that no one looks at it. Maybe it would be a good idea for more of the user base to try to encourage Steemit to start showing some of the promoted posts on more visible pages such as Trending, feed, etc. I know this suggestion has been made before but it has fallen on deaf ears.
If there was a way for people to use bidbots without it ever being directly profitable to do so, that would be the ideal scenario.
There isn't such a way, without declining rewards or burning a large portion of the rewards, possibly all of them (or strong downvotes which accomplish similarly). The problem is that you have to look at both sides of the transaction (vote buyer and vote seller). If the only variable is the vote price then making it less profitable for the buyer makes it more profitable for the seller and vice-versa.
I do agree with this.