Comment by Alupis

Most bots are either search engines (of all kinds, not just your google's and bing's), competitors, or academic/fun projects.

Of the three common types, only one has a serious interest in breaking captcha's - but they also have a serious interest in not getting too much attention by abusing your services deliberately. ie. if a bot is misbehaving, it's going to get our attention, and we're going to look into what it's doing, where it came from, who operates it, etc, and possibly take some action if appropriate or available. Those actions may not necessarily be limited to the technical space either...

This is just our experience. Other industries will have their own sets of challenges to deal with.