It will undoubtedly be an option for resort guests. But what do they do when they all arrive at DS? Pile on a Disney bus with all their luggage? Doesn't sound like much fun. If it truly is going to be a legit option for resort guests, there needs to be more than a few train trips a day.
I said this in another thread, it's just not smart thinking from Disney. The DME kept guests in the bubble, making sure they spent all their money on Disney property. Forcing them to choose alternative methods of transportation only encourages people to venture off-site at times.