I guess it depends how valuable that service is.

Right now, I can't imagine that being served content "more relevant to me" is so valuable as to warrant a corporation systematically recording my doings. My current reaction is: I'm perfectly capable of finding relevant stuff on my own, thank you very much.

But I'd be happy to be proved wrong.

Google is doing good today but it could be owned by anyone tomorrow. Even ARM Holdings changed ownership just this last year because of market/politics. So, replace Google with your poison - China/Russia/Your neighbor/Some bank/Wallstreet etc. Note that this is different from theft and security because in Google's case you willingly gave away the data.

Because, depending on your usage, Google can know more about you than anyone else, and can use that knowledge for much more sinister means than "serving more relevant content".

If by that you mean you will be more easily manipulated, then yes.

I do not want somebody else to choose what is relevant for me.