You are correct, but the station also has two main engines on Zvezda that are rarely used. It's easier and cheaper to keep these main engines ready for emergencies and let the docked craft perform altitude boosts.
As to the parent comment, burns are required because the ISS is at a very low altitude. It is still technically within our atmosphere and experiences significant drag from it. In addition, as the stations rotation changes, it's corrected using gyroscopes on the station. Once these gyroscopes have stored their maximum momentum in the flywheels they need to be released, and thrusters are used at that point to cancel out the momentum.
Something a lot of people don't seem to realize is that the station is moving all the time. Anything you do on the inside, such as exerting force on a wall or running on the treadmill (which had to be specially designed to minimize vibration transfer to the station) affects the station and must be cancelled out over time. The station moves as part of it's daily operation as well, such as when it enters [night glider mode](http://en.wikipedia.org/wiki/Night_Glider_mode) to reduce atmospheric drag.
Anything you do on the inside, such as exerting force on a wall ... affects the station and must be cancelled out over time.
Won't this cancel itself out over time anyway? If the astronaut kicks off one wall (acquiring momentum by applying a force), won't he just float over to the opposite wall and negate that momentum by applying a complementary but opposite force?
As to the parent comment, burns are required because the ISS is at a very low altitude. It is still technically within our atmosphere and experiences significant drag from it. In addition, as the stations rotation changes, it's corrected using gyroscopes on the station. Once these gyroscopes have stored their maximum momentum in the flywheels they need to be released, and thrusters are used at that point to cancel out the momentum.
Something a lot of people don't seem to realize is that the station is moving all the time. Anything you do on the inside, such as exerting force on a wall or running on the treadmill (which had to be specially designed to minimize vibration transfer to the station) affects the station and must be cancelled out over time. The station moves as part of it's daily operation as well, such as when it enters [night glider mode](http://en.wikipedia.org/wiki/Night_Glider_mode) to reduce atmospheric drag.