This is the question that Neil Paine asks at Baseball-Reference.

His argument is that the Giants cost themselves wins by not bringing up Buster Posey earlier and that the at bats given to Bengie Molina have hurt the team. The Giants could have a much more comfortable lead instead of figthing for every game.

Here is his argument:

You see, while Posey was spending 47 games in Fresno, the Giants gave the majority of their starts -- and 221 plate appearances -- to Bengie Molina in his stead. While he was keeping the seat warm for Posey's eventual arrival, the since-departed Molina turned in a truly terrible performance, producing a .644 OPS (well below the NL average of .715 for catchers) and registering -5 fielding runs above avg. behind the plate. Add it up, and Molina's 221 PA gave the Giants -0.4 Wins Above Replacement (WAR).

Needless to say, after Posey assumed the starting job, San Francisco's production at catcher skyrocketed. His 134 OPS+ is Joe Mauer among ML catchers, and his overall performance has been worth 2.6 WAR in just 372 PA. If you assume he would have maintained the same pace had he been starting all season long, and you give Molina's early-season PA to Posey instead, you find that playing Posey all season would have increased San Francisco's win total by 1.9 wins, which would give the Giants a far more comfortable 2½-game cushion in the West, as opposed to their current ½-game lead over the Padres. If San Diego comes back and wins by fewer than 3 games, you can point to Brian Sabean's decision to delay Posey's arbitration as a big factor in their losing the division.

It looks like in hindsight that this may have cost the team wins but things don't always turn out this way with "can't miss" prospects. Look at what happened with Matt Wieters in Baltimore. He was supposed to be the savior and his first experience in the big leagues was full of growing pains after destroying AAA like Posey had.

That being said I had some trouble just extrapolating Posey's Big league numbers backward like was done in the Baseball-Reference case but even using the actual minor league numbers and scaling them to the major league equivalent the analysis is the same. Using Posey's actual performance he could have been worth about 2.09 wins above replacement.(the math: wRAA = 10.38, Positional Adjustment = 3.626, Fielding = 0, Replacement = 6.933, total = 20.939 runs above replacement)

Those wins could come back to bight the Giants but for now with the information that was available at the time it was a borderline call and the Giants played it conservative hoping to save some future arbitration money and hope that Molina could hold his own and be somewhat valuable.

In hindsight it was probably the wrong call but when dealing in prospects it could have just as easily gone the other way. Let's just hope that those 2 wins don't kill the Giants.