If you have a multi-page or multiple visit conversion, then bounce rate is a great indicator of traffic quality. In you are converting on a single page, then bounce rate is not meaningful. That said, while using eCPM and other ROI-based metrics to govern bidding overall, you can use bounce rate in several ways:
If the bounce rate is over 70%, turn it off. If the term is bad enough, you may even want to delete the term and also add it as a negative keyword. For example, I push home based programs and discovered that any keywords with “home” were of ultra low quality. So “work from home” and “make money from home” are horrible. Having “home” as a negative keyword also improves traffic for “business opportunity” on broad match.
If bounce rate is 30%-70%, then you can use that as a quasi-lead. For example, if term A has a bounce rate of 40% has 60% of folks making it past the first page. And term B with a 70% bounce rate has only 30% of folks making it off the page. Thus, term A is delivering twice as many visitors per click as term B and could be bid up twice as much, all else equal. You’re still going to manage to a CPA, but if you don’t have many conversions or a low budget, this is a great early metric on whether that person will become a lead.
If the bounce rate is less than 30%, then something is probably working with the connection between your terms, ads and landing pages. If that term has high volume, place it in it’s own ad group and spin out more related terms. If your keywords are tightly grouped by theme (every term in that ad group means the same thing), then you should see bounce rate to be similar. Ignore bounce rates on low volume keywords, since you’ll see a lot of noise.