I have DDI so clearly Essential will get integrated into my game. But given that it involves no rules changes, I'm not entirely sure what it would mean to go essentials only.
It just means you are only using classes and races from Essentials. No different, really, than saying "PHB only" or "No Psionics" or any other restrictions countless home games have put in place before.
In this case, since it is designed for new players and thus designed to stand on its own even while being compatible with the rest of the game... some see this as a good opportunity for a more limited game. Thus, avoiding the potential 'option bloat' from the various sourcebooks and supplements currently available.