Justification

Readability.

Example 1

typedef struct xyz
{
   float x;
   float y;
   float z;
} xyz_vect;

xyz_vect a = {0}; /* RIGHT */
xyz_vect b = {1.0, 3.5, 6.7}; /* RIGHT */
xyz_vect c = {1.0, 3.5} /* WRONG: c is interpreted as {1.0, 3.5, 0.0} */

Example 2

typedef struct rotate
{
   xyz_vect hor;
   float    Rx;
   float    Ry;
   float    Rz;
} rotate_vect;

typedef struct foo
{
   xyz_vect    hor;
   rotate_vect rot;
} foo_vect;

foo_vect a = {{0}, {0}}; /* RIGHT */
foo_vect b = {{1.0, 3.5, 6.7}, {0}}; /* RIGHT */
foo_vect c =
{
   {1.0, 3.5}, /* WRONG: c.hor is interpreted as {1.0, 3.5, 0.0}. */
   {3.1415, 1.41, 0.707} /* Unknown if this was intended by the programmer  */