You can't trust the system browser to be up to date and secure or for it to render things how you want. You can not guarantee a good user experience unless you ship the browser engine with your app.
Yeah sure but I use most web apps through the browser either way so I'm already in "possibly incompatible land" and you can reasonably expect any user facing device to have an updated browser OR one specific browser in case of embedded. We're not in Windows XP software distribution times anymore.
Or actually learn how we use to ship software on the glory days of 8, 16 and 32 bit home platforms.
Now I do agree there are no alternatives for people that only care about shipping ChromeOS all over the place.