A couple of thoughts:

.1. If you sell/rent your SP, you're also bypassing others disagreeing with your SP being used in a manner they do not choose, as could be seen in the Ranchorelaxo and Haejin situation - you get the SBD from people bidding on your bot, or from wheover you sell it to, and people can't actually flag your content, because there isn't any.

.2. I'm sorry, but the following statement is so "no" on so many levels to me:

"Ethical concerns are a personal matter."

One of the core things about morality is that you can't go, "X is wrong/has a different opinion, and I don't mind," when it comes to moral issues, as opposed to say, liking a different colour.

Ethical issues in most schools of thought are exactly the ones where we say that we should be concerned what others are saying, or that it impacts us. You don't have to go all Kantian to go with, "This works so long not everyone does it. So long only one person does, they win. So long everyone does it, the system falls apart."

The question is whether this applies to vote-bots, but if it does, then you can see why it being an ethical issue, would mean it's not "merely personal." Yes, each person has to decide for themselves, if that is what you meant, but it is not merely personal.

.3. The concentration of power within Steemit is an issue. That people earn more money which in turn lets them earn more money, while not creating content is a byproduct of the vote-bot situation and the way Steemit is made.

.4. A thing most newcomers ignore while looking at how much bots are worth it is the essentially "free SP."

Suppose you pay 5 SBD for a vote, get 12 STU vote back. After curation, assuming it's about 20% cut (it often is), you end up earning 4.8 SBD, which is a "loss", but you also get $4.8 USD worth of Steem, and end up making more than your share.

From a perspective of money-making, so long you find bots whose makers ensured would not end up losing you money, you have no reason to not buy votes. Except it destroys the trending section (look at the Promoted section, or you should've looked before everyone there started using bots to see the point), and the economy is disrupted by it majorly. As nobody actually ends up earning anything if everyone uses it (because it'd end up with the same reward pool split, percentage wise), except the people offering the votes.

And that's what it boils down to. Should everyone use the bots, the only ones making anything out of them would be the ones selling the votes.

Should everyone but you use the bots, you'll end up losing even more.