To me the metrics make it a level field for comparing. Watching one shortstop 150 times and another one 6 times shouldn't lead anyone to conclude whose better.
But the best thing about metrics is they made me and a lot of other people look beyond errors and fielding percentage, which was a very common method of evaluating defense, and easily the most flawed.
Eye Test defense is OK, but it has far more flaws than metrics do...