I recently noticed a Chrome extension I had installed—A simple utility—was loading Mixpanel into every page I visited—whether or not I was using the extension. Having published a popular Chrome extension in the past and receiving random solicitation from people offering to pay to track our traffic across the web, I know this is an industry.
This is an obvious threat to privacy and security, but also a threat to a pleasant browsing experience due to bogging down web pages. Both are terrible for the open web.
Take the time to be aware of the issues if you are not already paying attention.