It encourages infill within the city boundaries, which appears to make land use more efficient. The New Urbanists would go even further, creating an order so you have seamless urbanization. The New Urbanists don't like what they perceive as disorder. It's important to realize that Neo Traditional planning is focused on new development. In order to get these urban-design concepts through a city council, the restrictions have to be on future generations, because people living there today won't vote for it.
Planners are on a continual search for a better community.
The planner's job is to make communities more liveable. But they completely mis-diagnosed what the notion of community is. What they should do when they look to the 19th Century suburbs is recognize that those suburbs have been in place for generations. When you look at a modern suburb, you find that they are very new communities, with only one or two generations of experience. But look at them after three or four generations, and you'll find more sense of community.