Email, of course, contains tons of personal information. If linked to a user's identity, it could clearly be used to inform advertising. However, emails in aggregate probably don't give you any more useful advertising insight than the Web corpus itself, which Google could obviously use if it wanted.

Further, why would Google need to log random bits of WiFi payload data to extract a relatively small amount of email when they operate Gmail?! If the decision to purposefully violate people's privacy had actually been made, wouldn't it be easier to look at Gmail than to use Street View?

There would be more data, better data, and you'd have it more quickly than if you sent a bunch of cars to drive all over the world, collect a bunch of extraneous crap, then wait for them to come back, and screen it for something useful.

Frankly, that's absurd.