Why does the Arduino UNO have a separate power port if USB also powers it?
Can you power the Arduino off a USB cable alone, without using the
  regular power connection?


You can use the USB cable alone to power the board.


  If so, what's the point of the additional power connection: to provide
  extra power?


You can use either to power the board. By using just the power plug you can make your Arduino portable by using a battery box or power it via a wall plug.


  Also Why not just have two usb ports one for power one for connecting
  to the computer, or a micro usb.


Only ONE USB is needed to power and program.