And why should we trust you (or Google etc) with our data? Perhaps you (they) will exploit our ignorance and manipulate our preferences?

Not everyone in the world can be assured their human rights and exposing personal information can lead to persecution. Therefore, there is a risk attributed to this data collection, at least for some. Not everyone is willing or able to take these risks.

While I can agree that this data may be useful for both customer and advertiser, its potential misuse should not be ignored. And when these networks deny the use of pseudonyms, they force their users to take (unnecessary) risks.