From a stats stand point, Jason Bay hasn't made an error yet this season. He has 6 assists on the season, only David Dejesus has more. He is 3rd in MLB with a 2.26 Range Factor and 8th in MLB in Revised Zone Rating. Consider all the great diving catches and out-of-zone catches he has made it looks pretty convincing that he is above average.
I do find fielding stats to be fairly flawed. For example, Adam Dunn is ranked 4th in MLB with a Revised Zone Rating of .899, but clearly, he isn't a great left fielder.
However, try coming up with a list of 10 or more left fielders that are clearly better than Bay. It isn't all that easy.