I'm going to go against the trend here and suggest that there are a number of situations where a high brace is a much better and more flexible tool than a low brace. I will agree that there are places one definitely doesn't want to use it. It's generally not a good idea to use one playboating, in particular when side surfing a hole. That's where most people have tweaked a shoulder using a high brace.
The places where a high brace is the most appropriate tool is where one has to cut through a bunch of currents that are rapidly changing direction. Examples include turbulent eddy lines, in steeply dropping slots which change direction, crossing boils and seams, cutting through breaking lateral waves and puching through the apex of a two converging laterals.
In this type of situation, what often happens is someone goes in with a low brace. When the current flow changes direction, the paddle blade which was providing support turns into a liability and pulls them under.
To be used properly as a support in the type of situation I just described, the paddle blade should be placed well forward of the body, several inches away from the side of the boat with the shaft of the paddle close to vertical. From this position, the brace can be converted to a draw stroke, a forward stroke or a pry stroke.
A good practice technique for this is to practice moving your boat sideways using sculled draw strokes to go one way or sculled pry strokes to push the boat the other way.
To see high braces used properly, watch either good slalom or C1 paddlers in heavy water.