I think the key phrase here is 'usability for bots'. If you recognize bots as an important customer (after all, they bring over most of your new traffic), then it makes sense to optimize things for them to find.
That said, somewhere in between of not writing your whole site in Flash and creating a link farm, things start getting slimy and your arbitrary line will likely differ from, say, Google's.
I think the difference between the best SEO agencies and the majority is that the best continually run experiments to try and understand what Google is doing. This takes no little skill and a lot of time and effort.
The rest just read and regurgitate the results of those experiments when the experimenter decides to release those results (ie when it's not too much of a competitive advantage).
