The gutter doesn't take up space. If it wasn't there, then the tiles would have to be larger to fill the horizontal width of the screen. Since they are squares, this makes them take up more vertical space. The screen could only show 3.5 rows of tiles that way, instead of 4.3. And scrolling takes longer. So removing the gutter wastes screen real estate.
Now that WP8 has quarter-sized tiles that must be useable, it makes more sense for the medium size tile to be so huge, and the smallness of the new tiles will compensate for the screen real estate wasted by the lack of a gutter.