I don't like the "persistence" of Google either, but... if you want to use their package of services (Plus, GMail, others...) the stakes are just going to get higher and higher.
Today, it's a cell phone number for the "benefit" security. Tomorrow, they will ask for a SSN in order to use Gmail.
They offer free services for one reason, and one reason only... and it isn't because they like giving away stuff.
Google is offering "X" service so they can bargain your data out of you, more and more of your data. That, IMO, is the reality of the situation. I would challenge that most people don't understand fully the bargain they are getting.
Google knows most people don't understand this bargain.
Now, that said, do they offer value in the services they provide in exchange for this data? Sure.
If you want that value, they will either:
1. Get your data, and quit asking for it.
2. Not get your data, but keep asking for it, as many times as they want, because it's their sandbox.
And here's the kicker in my opinion, millions of people are going to keep signing up for and using their free services, and keep offering their data in exchange for using their services.
The Google world just keeps on turning...