Here is my take on this:
A top witness shall be able to run a bot promotion service. Why not? It is a free market, if we impose rules on what one can do or not we will dive into another dangerous black or grey region.
I think that @therealwolf is fair game and even if you can buy and sell votes offline on his platform, you still see that the vote is done via his site. He could hide the sold votes via steemconnect and nobody would be able to trace them.
Before the bid promotion bots, there was a huge offline vote selling. No one could tell if a vote was paid for or not. At least with this services we have an incentive for investors to put money into Steem. Real investors look at services where their stake grows without investing to much time in it. The higher and faster the returns, the better. This is basic economics. Most high rollers don't have the time to lose and an bid promoting bot provides them the appeal to buy Steem. I see that most people who complain about the bots are poeple who never bought Steem and are in constant Power Down modus. This ones are the real killers and the ones who put pressure on the Steem price.
Steem is not only a blogging platform, which a lot of people don't udnerstand. The Steem blockchain has many more uses, one of the most important is being the fastest and cheapest blockchain in the world. Compared to BTC, it has 0 fees and can be used as a real world currency. Imagine to pay in BTC at the supermarket and wait 30 minutes so that the payment comes through. Good luck with that.
Before bid promotion bots have been here, the trending was also shit, pardon my French. There have been the same accounts over and over again.

Also taste can't be defined, what for some might be a highlight for the other one might be trash. I see posts that promote drug consumption. A lot of people will cherrish about it, others will say that this shall be banned.
I see a lot of posts with conspiracies, which make me laugh be cause, like the ones about flat earth. I would rather flag them then give them a vote and this is only because my taste is different from others, and this is good so. The truth lies in the eyes of the beholder.

Coming back to the question: Shall a witness be able to run promotional bots on Steem? Of course. There shall not be a limitation in this. Luckily we are in a free world where can decide and have to live with the consequences of our choices.

We have a lot of top50 witnesses that don't post anything, or do anything else than having the server plugged in, they've been here at the start and received the votes. Are this ones better than the guys who build some tools that incentive investors? I don't think so.

Here is another question: What must a witness do? The main purpose of a witness is to secure the good functioning of the network, or am I wrong here?

As a disclosure, together with @catalincernat we run the witness @ro-witness.