Not only do lots of people still visit google.com, but the types of people who still visit it are precisely the types of people who need to learn about this law. I suspect those of us savvy enough to use a toolbar are disproportionately more likely to already know about and oppose the bill, while the kinds of people that go to google.com to search for 'facebook.com' are disproportionately more likely to know nothing about it yet.
The kinds of people that go to Google.com are also the kinds of people who don't fully understand the repercussions of a law like SOPA. Google not working would drive home the message to them.
They could learn about SOPA from an informational page like the one Wikipedia is going to put up during the blackout.
No, the kinds of people that go to google.com are the kinds of people who would assume "the internet broke" if google.com looked dramatically different when they loaded it. They may even need someone to point out to them that Google is actively taking a stance on it, despite having seen the page.
