It seems to be becoming more and more clear that 'bid bots' are becoming less and less popular - yes they drive investors to Steem, but sometimes at the expense of driving quality content. Often these bots have no quality control and so anyone can upvote themselves to the trending page. There is now becoming more and more people prepared to flag 'bot abuse'.
On the other hand bots which have a community in mind and have some form of quality control are great for growing the community and helping under rated authors. Some bots that I think really benefit the community are:
@Steem-ua - A bot that upvotes users that delegate - higher upvotes can be gained by having a higher Steem-UA score - gained through activities like engaging with the community - a great way to encoruage users to grow, and reward them for it
@busy has a bot that upvotes users who use the #busy tag - again it has growing your account in mind - higher upvotes are reserved for those who have more followers with higher organic Mvests (SP)
Bots like this are helping out the community and encouraging users to grow, while also attracting investors to delegate.
It seems that bots are a nessasary evil here, but in the long run I think there is going to be a shift away from bid bots
There are actually different kinds of bots, so it would be difficult to say without knowing what kind of bot you want to know about.
The most likely option is that you're referring to bid bots. Bid bots are designed to allow a person to "buy" the vote of another person in exchange for slightly less money than the vote is worth. By sending Steem to a bid bot, a person can a portion of the vote from an account that has a larger vote.
For instance, if a bid bot has a vote of $10 and four people each send 1 Steem to the bot, they should each get a vote for $2.50. 25% of the vote is taken for curation, so each person is left with $1.87. With the price of Steem today, each person would get 6.2 Steem back if they posted 100% powered up. This number is exceedingly high and most bid bots have switched to a specific percentage return on investment (ROI) so that bidders can't get highly-profitable votes.
Most voting bots are set up so you send a certain amount of Steem and get a vote back that is a predetermined amount larger. So you could send 1 Steem and get a vote worth $1.4. After curation, the vote is worth $1.05, so you get a 5% ROI. This is assuming the price of Steem doesn't change drastically within the period between when you buy the vote and when it pays out.
Many voting bots do not give out very high ROIs, and due to the amount that voting bots are used, it's debatable if the use of bots is useful. It takes a lot of resources from the general pool and only distributes a small amount of gain to those who use them. The main gains are going to the voters who sell their votes. Due to this (and other issues) voting bots remain controversial.
Voting bot owners normally say they're giving people a chance to get exposure for their posts. They also say that it's permissible by the blockchain so they're doing nothing wrong. Some people say they're running a business on the blockchain and that's progress that will help with mass adoption of the Steem chain. Time will tell.
Another type of bot is a notification bot. These send messages to a user when a specific criterion or criteria are met.
Another type of bot that can be used is an auto-voter bot. These services allow a user to choose authors they want to reward and set established vote values and times per day that author is rewarded. They don't have to be present to cast the vote manually, the bot takes care of it.
tl;dr There are many kinds of bots. Each has a use or purpose. Some people disagree about the benefit of using some of the voting services. Each user must determine for his/herself if voting bots will be used. No controversy has been noted with notification bots.