I'm going to copy-paste from an older comment of mine (https://news.ycombinator.com/item?id=22332136 https://news.ycombinator.com/item?id=22332136) that I think

I'm going to copy-paste from an older comment of mine (https://news.ycombinator.com/item?id=22332136 https://news.ycombinator.com/item?id=22332136) that I think captures my opinions on this question: Client-side tracking, if you need any at all, should be a) high value, b) aligned with my goals as a user, and c) as respectful as possible of my privacy (eg. anonymising values, only taking what info you need). The b) condition there is most nebulous - I'm mainly thinking of things like reporting client-side javascript errors. This is aligned with my goal of your site being bug-free so I can use it better. Another example would be an (opt-in!) recommendation system that I find valuable. What would NOT be an example of this would be tracking of my actions on the page in order to optimize the chances that I'll engage with the content. Engagement is your priority, not mine.