I've used Ghostery for years- and highly recommend it for anyone involved in marketing.

The obvious use-case is to not inflate your clients' data. You know- when you are working with a small-traffic site and your visit is one of only a few. Don't inflate your client's data! Use Ghostery to exclude yourself!

The most valuable use of Ghostery, however, is to see what tracking systems are involved on a website. I use Ghostery as part of my audits- to see what a client is using. Sometimes even they don't know!