The apps on your phone would not have the ability to suck up isolated pieces of data about your usage and combine them into a single profile with your age/gende

The apps on your phone would not have the ability to suck up isolated pieces of data about your usage and combine them into a single profile with your age/gender/etc... if they didn't have a set of shared identifiers that they could use to associate that data across multiple sources. It seems very relevant to me. The larger privacy problems wouldn't be quite as bad if there was no way to tell what device an app was running on or who was using it. That seems like it would limit data collection in a pretty significant way.