But I agree with you that it's very different from anything they do. Also, too vulnerable to piracy or cloning. Aaand it may not be feasible to run their bots in a weak machine.
Gmail ad targeting isn't a bit of JavaScript that can run in real time in the background. It's a series of huge map reduces touching data sets larger than any client computer could store. Just indexing a pre-prepared list of mothership ads would result in a horrible user experience (far poorer ad targeting).
Google has a vested interest in only showing you ads you might be interested in; it doesn't work as well as it should, but it works magnitudes better than the state of the art 10 years ago.
And if the protocol is sufficiently closed to prevent open-source clients, why would anyone use it at all? Wouldn't that be the same as the XMPP->Hangouts transition that infuriated everyone a few weeks ago?