Anti-flyers have, throughout several threads said that players will always take the path of least resistance, which is really applicable to humans in general. So if Blizzard wants to encourage WPVP, bring back danger, slow down content consumption and get people interacting in the world, it seems silly to overlook the fact that people can avoid all that by sitting in town all day. But yes, we don't know exactly how many people never (or infrequently enough to be statistically the same) leave town, but I suspect its fairly high.
I wouldn't be surprised to see both dungeons gated by completion of zone story quests. IE: the summoning stone and instance don't even become reachable until you have followed the story up to what it takes to defeat the hordes of enemies in front of the gate. This would make it impossible to sit around in town and dungeon all day before you hit max level, and would achieve all their goals as well.