Once Chrome becomes stable on Android and it replaces the stock browser, I don't know why you'd still want ChromeOS on a separate device, or say even dual booting with Android. Would there really that huge of a security advantage to do that, instead of simply running Chrome from Android?

Because besides the security issue, I don't see any advantage at all for ChromeOS in that scenario. And even so, the convenience of having Chrome in Android most likely trumps whatever security advantage ChromeOS has over Chrome on Android.