There are many uses for bots other than just auctioning off votes. Quality content is also a very subjective topic. I'm not debating either of those subjects here, but rather moving the preferred payment currency on Steem to be STEEM rather than SBD.
As for the Witnesses; many of the witnesses use bots to automate various tasks which may include, but certainly isn't limited to, using some of their voting power on autovotes for various communities, or also selling votes via a bidbot service. Some bidbot owners are actively monitoring their bidders and blacklisting abusers. @sneak-ninja is one such bot.
I'd be interested if someone could tell me a way in which a bot can be used to curate quality content. I can't think of a way for that to work if the bot is the one doing the upvoting.