Second, if you worry about your file getting in the wrong hands (either now or later), then paying them won't help with that. To provide the services they provide they need to have that file.
Paying at least makes explicit the bargain we're striking. As opposed to Google offering me something for free, then having every incentive to suck as much money out of what they can glean from me, I'm instead simply paying them for the true cost of the services I consume.
And, more importantly, if they roll out a new service and you start using it, you'll immediately start thinking about the cost of using it. Or people will start to "demand" more for their money. (Things like support.)
But I do think you're right about the "branding" issues. They don't want to broach that can of worms because it reminds people that things aren't free and Google is making money to pay engineers and shareholders somehow.
Point being: my data is worth $X to Google. Based on this, they develop and offer me services, using $X to fund development.
The monetization creeps me out. Mostly through opposing my privacy goals and Google's profit goals. As with another poster, I'm worried about my Google file being in the hands of 20-years-from-now-Google, notsomuch Google of today.
So I'd rather just pay Google directly, call it even, and be certain that my data wasn't in the "pot of gold" pool that everyone's eyeing for whatever new monetization strategy occurs to a Google exec.
And it sucks, because I'd love to use Google Now, Home, etc. I just don't trust them with those data streams.