Tin foil hat on. It came to me last week.
Google has recently released Project Fi. A project/product (is project a codeword for beta now?) that will allow seamless transition between 2G/3G/LTE and *WiFi* for increased coverage and strength.
Project Fi is bandwidth charged, independent of data link being used - so while the underlying carriers (T-Mobile & Sprint) may charge wholesale for data, google will effectively get the bandwidth at "Google WiFi" for free - meaning that the data charges are a lot more profitable when going past a Starbucks, in NYC, etc. Although unproven, google might actually have meaningful alternate revenue sources from this model.
This is not fundamentally different than Xfinity's wifi sharing - except google is going for the Free WiFi in the Starbucks, NYC, etc. B2B is a lot more pragmatic, and a lot easier to enable.
Now if only Project Fi worked in a phone that was in the $2-300 range, I'd probably give it a go. But the Nexus 6 is too big and too expensive. Hopefully the invite won't expire.