For example, you may leak information about sites recently visited, updates applied, etc. if you have a local proxy cache and subsequently look at response timings.
That depends on what you mean by privacy. We share information, sometimes sensitive information, with other parties all the time when we interact with them. I believe the essence of privacy is more about being able to choose when and how and for what purposes information is collected and shared and used.
I can't make a purchase using a credit card without the card company at least knowing who I'm paying and how much money I'm giving them. There's not much point going to see a doctor if you're not going to discuss your medical situation with them. If I go out to visit friends, someone passing me in the street is going to know where I am at that moment in time. That doesn't mean anyone else needs to know any of those things, or that they need to be used for other purposes or correlated with other data.
In any case, with a lot of information sharing that is going on with software and networked systems these days, it is far from clear that many of those "potential optimizations" are actually in users' interests at all. Obviously some facilities do need to analyse relevant data sets to make useful predictions -- personal assistants like Siri and Cortana, say, or recommending new material that is similar to what you've accessed before on Amazon or Netflix. But even there, the limitation is often that the technology isn't powerful enough to do the same things locally yet, not that the organisations running these services inherently need to know lots of data about you.
This is the core of the EU data protection principles. It's a very concise way of expressing things. There are two limitations:
- figleafing: the "cookie law" problem where everyone is made to agree to a useless dialog box, supposedly signing away their privacy in order to look at any web page with ads on
- it conflicts with the very strong American free speech principles, in which you can say anything you like about anyone on any basis. Privacy enforcement necessarily means silencing people talking about other people. The bad end of this is UK libel law. It's still present in the US "product libel" laws, although fortunately "ag-gag" was recently struck down.
The platform should hopefully be 100 % trustworthy (from an "it's free software so I can inspect it" point of view), as long as you do not choose to use a non-free graphics driver.
1. https://blog.torproject.org/blog/mission-impossible-hardenin...
The convenience of one device is a big sell though, plus I think a device with a built in cellular modem is more fairly called a "phone".
It reminds me of the argument that "of course NSA spies, that's what it does" completely merging together the spying on dangerous targets for national security with the spying on every single person on Earth and for economic, blackmail and so on purposes. Reality is more nuanced than that.