1. He calls a good game
2. The Giants felt that Buster Posey was not ready
3. Offensive production
Offensive production! Wait, really...offensive production:
To me, this move represents everything that is wrong with the Giants right now. Instead of pursuing a player who could dramatically make the Giants lineup better, Sabean opted to spend his money in multiple places hoping to make the Giants lineup deeper and more potent. But even after adding Aubrey Huff and Mark DeRosa to the fold, Sabean realized that he still need a middle of the order hitter, who could give the Giants the punch they needed.
In addition, the catching market was so poor that Molina, who was considered to be the top catcher on the free agent market, was not likely to land anything more than a one year deal with an option. Once Molina decided to take a one year deal, he fell into the Giants lap. The Giants have gone after veteran players over the past few seasons with short contracts and the one they gave Molina is no exception.
Because of his limited resources and strange affinity for aging players with low OBPs, Sabean went after Molina. Ultimately, I think the Giants could have allocated their resources better because I don't think Huff, DeRosa, and Molina will improve their offense that much. But it's clear to me that the Giants value Bengie Molina a lot more than most teams do.