> Do the people who use Chromium really sign into their Google account? If that's the case, you can just as well use Chrome itself.

I really don't understand why this is so surprising, if not Google's server with a google account, where?

And yes, you could just as well use chrome, but Chromium is entirely open source, while Chrome is not. It may not matter to everyone, but it matters to some people. So yes you could get the same functionality from Chrome, but it has different non-functional characteristics which matter to some people.

> My assumption was that Chromium users would want a browser without all of Google's cruft.

The "cruft" like bookmark syncing, password syncing, etc, has to come from somewhere. Google was is good a place as any to me.