It is not *preferable* to block (generally one sword will be struck against another at a right angle, typically creating an "X" shape) with the edge of a sword. But a swordsman will find themselves (during real combat) tricked off guard at times, and will have to use the edge to save themselves from being struck down. Blocking with the edge is simply the fastest, most direct, and strongest (alignment wise) defense against an incoming cut in favor of blocking with the edge is the considerable leverage you are able to apply against the incoming strike. The grip is technically weak in relation to the direction of the incoming strike if a block with the side of the blade is attempted. 

 

 

 

 

 

 

 

 

 

 

The edge will likely chip/gauge if used to block. Technically, it could be said that an advanced swordsman would be less likely to be "stuck" in a position where a block is necessary, and as such would be utilizing more refined movements such as diversions and parries.