So to start with, a disclaimer: I carry some baggage here, from working occasionally on 10+ year old GWT app that's developed into quite a tangled mess.
Having said that -- this sort of thing doesn't seem like an obvious win to me. The notion of "front end" and "back end" represents a real-world distinction that you want to pay attention to. Making it too hard to see, or too easy to communicate across, brings some risk of losing track of which is which, and putting logic that belongs on the server into the client or vice versa. Or having the two communicate with each other much too often, which may not cause any noticeable problems during development, but, in production, can cripple the site for users with higher-latency Internet connections.