From what I can tell the grid object type is just a subdivided plane. What is the difference between adding a plane and subdividing it, and adding a "grid"? If there is some hidden difference what is it?
I don't think there is any difference, other than it's slightly faster to add a grid than to add a plane and subdivide.
From the wiki:
A standard grid is made out of n by m vertices. The resolution of the x-axis and y-axis can be specified in the context panel in the Tool Shelf which appears when the object is created; the higher the resolution, the more vertices are created. Example objects that can be created out of grids include landscapes (with the proportional editing tool or Displace modifier) and other organic surfaces. You can also obtain a grid when you create a plane and then use a subdivide modifier in Edit mode.