And every time someone says "but how does it harm me for advertisers to know everything about me?", THIS is the answer. It's not just "finding things you might like to buy". It's "auctioning you off to vendors based on how badly you need the thing." The market is an information game and if one side knows more than the other, then they have an advantage.
Really think it'll stop at helpfully suggesting a nearby gas station, when Station A can just pay $1/customer extra for GM to send people their way instead of to Station B who are charging $1/gallon less but haven't paid extra to GM?
Really think it'll stop there, when GM can get an extra $1/customer by telling Station A that you're critically low on gas and can't make it to another gas station, so they can gouge you $5/gallon and you have no choice to pay it?
Allowing your personal information to be indiscriminately mined and sold is bad for you and this is why.