I don't think ChromeOS can be useful as a tablet interface, just like I don't think Ubuntu would be useful like that, but I could see it somehow dual-boot, or easily change between the Android and ChromeOS mode when being docked to a keyboard, with ChromeOS being more optimized for that kind of usage.
I totally agree with you. I can see myself using something similar: a tablet when I'm travelling, relaxing, or watching a show, dual-booting into Ubuntu and plugging in a mouse+keyboard+monitor to get work done.