No defensive metric is perfect, nor will one ever be, because every metric relies on an observers judgement. Was that an error or a bad hop? Should the 1B have made that catch or was it a wild throw? That said, if 3 or 4 metrics all paint the same picture, then it's pretty safe to assume that the picture is pretty valid, and statistics aren't biased like people's memories are.
Take Jeter for example. Those of you that watch him everyday are influenced by the spectacular, and I will give Cap'n Intagibles credit for being able to make the spectacular play. Every time he does the Statue of Liberty throw from deep in the hole, or dives into the stands, or ends up in death valley on a pop up, everyone goes "Wow, he's incredible in the field" and they forget the 2/3 errors he makes the same week. Rate, Rate2, FRAA/R, ZR, UZR, RF, and FP have him substantially below league average for his career. No, those stats aren't perfect, and they are influenced by who is on the field next to you. Jeter's two best statistical years, the last two, were when ARod was scooping up balls next to him.