They wouldn't need to install anything new on your computer to start tracking you in more detail or building a user profile on you, they could just start doing it invisibly behind the scenes on a server someplace. That's a big deal, because even though you're trusting them to administer your device, if they did start pushing out spyware, there's a good chance a security researcher would notice it. But there's no way for us to know what Apple does with this data once it leaves our devices.