If you act on the internet like you would in real life, it doesn't seem like such a big deal. How is getting relevant ads a bad thing?
During this time, I let my girlfriend borrow my laptop one night. Each site that she visited was riddled with ads for engagement rings.
Next thing I know, she's giving me the twenty questions about why I'm looking at engagement rings.
Some things need to be kept private even if they are benign.
Until it was too late. Until your spam filter thought it wasn't spam. And on that day, the privacy geeks didth retort: "told ya so."
For example, a supermarket chain might use an aggregate of purchase histories (i.e., products most often purchased together) to influence product placement in the store. They won't re-position products for each customer as they walk in the door, though... Online, this can, and does, happen.
It might not seem like a big deal when it's useful. However, there will be many instances of people trying, and failing, at creating a useful product. I think that's a valid point of distinction: when people fail at adopting new technologies right now, the results are mostly harmless. If they fail with machine learning, there could be some major privacy concerns.