Google's Macondo Street View team cannot seem to get the right combination of top kill or cap to fit on its MAC spillage. Your MAC is not like a house number (which everyone knows and are used for many purposes), MAC address is scoped to one use. There's no harm in collecting MACs, the hell you say, there's a number of evil emergent cocktails that can come out of this. Its not so much the MAC itself, its the association of the MAC and the gelocation and time - combining something unique like MAC with geolocation.
This looked like a rogue team (or as Google put it last week a "rogue software engineer") until this shocking announcement that Google is patenting (emphasis added) - "The invention pertains to location approximation of devices, e.g., wireless access points and client devices in a wireless network."
It seems pretty obvious that any number of permutations of problems will result by combining private client data and geolocation. Maybe Google books should scan a copy of J.C. Cannon's book "Privacy: What Developers and IT Professionals Should Know" and Stefan Brands Primer on User Identification. In both works you see the risks of promiscuously mixing identification cocktails and the unexpected leakages that result. In addition, what does benefit to the user who is being spied upon does all this spying create?
Increasingly, we are faced with questions like - do I want to turn my whole life over to Apple and their shiny happy Disneyland experience and be utterly locked in down to the hardware/data level; or Google who will spy on me with the NSA and then sell me ads? These choices are like being asked which Menendez brother do I prefer.