Heh, it seems like Google is becoming the new MiKKKrosoft, because OMG EVIL they're taking over the world!
Where some see "ulterior motives" I just see "business model," I guess. They're a web-centric company. It makes sense that they would produce a web browser.
And however reluctant they may be to allow ad blocking with Chrome, they're canny enough to know they'll get all kinds of nerdrage aimed at them if they try to keep 3rd party stuff from being developed for their open-source browser. It's just a matter of time, someone will make it, I'm sure.
But then again, who knows? Maybe it'll be like homebrew-hacking your game console... like "sorry, you can't update to the latest version, you have unauthorized extensions installed." Even so, someone will probably come up with a workaround. Want to run Chrome with no ads? Try Krome!