PappyJ:
Minimum Sector Altitude - The lowest altitude with may be used which will provide a minimum clearance of 300m (1000 feet) above all obstacles located in an area contained within a sector of a circle of 46 kilometres (25 NM) radious centred on a radio aid to navigation.
The "may be used" doesn't apply to the U.S, except in an emergency.
Further, as you cite, they provide 1,000 feet of obstacle clearance. In the Designated Mountainous Areas (DMAs) of the U.S. the pilot is required to maintain 2,000 feet off published routes, and MSAs are not published routes or sectors under FARs Part 95 or 97.
The rules vary all over the map (literally), but what I state is for FAA-dom.