It would be impractical and unsafe to keep our cities and streets completely dark, but it would be great for astronomy. Streetlights are needed and the type that astronomers really like is low-pressure sodium (LPS).
The big advantage of LPS for astronomy is that it puts out a very limited range of color. This limited set of colors can easily be filtered out by astronomers, who then can use the rest of the color spectrum to study the universe.
Here is an example of what I am talking about. Below are two street lights. On the left is a new broad-spectrum (aka white) induction currently being tested by the City of San Marcos, CA. On the right is a Caltrans LPS light over Highway 78.
If you take the light being give off by each fixture and pass it through a diffraction grating (something like a prism that splits light into its component colors) you can see this:
The spectrum on the top is from the white induction light and just below it is the spectrum from the LPS light. Notice that the induction light has essentially a full rainbow of colors, but the LPS has a very limited number of colors. So you can see why it so appealing to astronomers. There is very little color to get in the way of astronomical research.
Here is a close-up of the spectrum of just the induction light (with the light itself on the right):
One of the draw backs to astronomy is the all of the blue light in the spectrum. The short wavelength of blue light makes it more likely to reflect off of the ground and up into the sky. So even if you have a good light fixture that directs all of its light downward you'll get a brighter sky than if you used LPS lights.
The limited color in LPS light does pose a problem for anyone that needs to accurately determine color under just that light source. That, along with potential energy savings, are part of the reasons that many cities are now looking toward new lighting technologies.
The energy savings question is a bit muddled though. Low-pressure sodium is still the most efficient lighting source around. It is about three times more efficient than the best LED lights around these days. But the extra colors in a LED or induction streetlight make them look brighter to the eye than a LPS streetlight of the same brightness. This can allow cities to actually use dimmer lights whithout people even noticing.
It is vitally important that if existing LPS (or even high-pressure sodium) streetlights are replaced with new induction or LED lights that the new lights be of lower lighting levels. Without that the sky will just get brighter for everyone and streets will be over illuminated. In the end we would all lose.