I couldn't care less if Apple knows what I use the iTunes Store for, but I would care if they made me store all my data and apps on their servers as Google wants users to do.
Think about all the information you have on your hard drive - browsing history (which Google most likely already knows quite a bit about), family photos, videos, films, music, accounts, and calenders, to name a few. If you were using Google's OS, they'd have data on all of those things - they'd know what you spend money on, how much money you have, what your family look like, and where you'll be in the future as well as what Apple can get from iTunes Store habits.
Scared yet? Lots of mobile devices are also location-aware, so they can find out where you live and where you go out - and it's often said you can find out who someone is from that data alone, but add that to all the other information Google will have on you by that point and it's a privacy disaster!
So, I am very opposed to advertising companies storing all my data and software thank you very much.