Bensonwood have used horizontal corner braces in some of their buildings (or at least one). Take a look at the cover image of "The Timberframe Home". The only circumstances that I can imagine them being useful for, would be a structure with no sills, where the posts could (in theory) slide around parallel to one another. In a building which is anchored to the ground, the ground should (to my mind) resist those racking forces quite well!
