I have 2 issues with my data being used.
First is personal: I have nothing to hide really but it's quite frightening to see the amount of data these corporations (and the state) has about you. They probably know more about yourself than you do.
Your cell phone manufacturer knows where you are at any time of day / night and who you hang out with, facebook knows what sites you visit (provided they have a like button on the page),....
Then on a more philosophical stand point, it is a clear denial of one's right to privacy but more than this, the problem is that it can potentially lead to have you locking yourself into a line of thought when internet should offer the opposite:
Google will learn from your search history and the sites you visit and will adjust the search results accordingly to offer you what you want to see...
That means that if you always click on "republican" news sites, then after a while, google will give you only right wing results, progressively disconnecting you from the other side of the story.
For me, internet should be a place where you see other people's POV not something that is tailored to please you
