While both deals were bad, the two contracts came from very different places. Aaron Rowand was the benefactor of some very good luck. He had always been, and continues to be, an excellent defensive player, but in 2007 Rowand established career highs in just about every category, hitting .309/.374/.515 for the Phillies. This wasn't completely out of the blue - Rowand had been the major piece in the White Sox/Phillies trade involving Jim Thome, and had a very good 2004. However, he followed this up with two sub par seasons and by the time he hit free agency he was entering his age 30 season. An out of nowhere season for a player entering his 30's usually means trouble is coming for whoever signs him. The Giants ponied up a 5 year, $60 million contract, hoping to minimize the offense blow of losing Barry Bonds. If Rowand could continue to hit while playing excellent center field, he could certainly help to keep the Giants afloat.
Sadly, it just wasn't meant to be. Aaron Rowand showed that the poor 2005 and 2006 were much more representative of his true talent than his impressive 2007 season, and Rowand struggled in all four of his seasons with the Giants, and according to Fangraphs, he never was an above average player on offense. Despite that fact, Aaron Rowand was an above average player for the Giants, just nowhere close to his $15 million/year price tag. Thanks to some spectacular defense in centerfield, Rowand logged positive value overall for all four years of his contract. Ultimately, Rowand isn't a bad baseball player. Based on Fangraph's calculations from his wins above replacement, Rowand was worth $21 million in his four seasons by the bay. His ability to hold his own against lefties and play all three outfield positions well makes him a great fourth outfielder. Unfortunately, the Giants were looking for a star and had a hole to fill in center field. The funny thing is, Rowand may still play a role in the playoffs this season. Several teams, including the Phillies and Red Sox, are a little thin in the outfield and could use his versatility and I imagine we'll be seeing Rowand somewhere in October, after the Giants are sitting at home.
The Miguel Tejada deal wasn't nearly as bad. It is almost impossible for a one year deal to go horrendously wrong, especially at a price tag of $6.5 million. With Tejada, the Giants again were looking to an older player to contribute, despite warning signs about what was coming. In 2010, Tejada was terrible for the Orioles, but had a bit of a bounce after being traded to the Padres, hitting for a .730 OPS. Nothing incredible, but in Petco Park, that isn't horrendous. The Giants stepped up this offseason, hoping he could building on what he had done in San Diego. Splitting time between shortstop, where he's been below average defensively for several years, and third base, where he has been a solid defender, but his bat looks even more inadequate, Tejada limped to a sub .600 OPS and was worth LESS than replacement value.
It is a shame the Giants won't get a chance to defend their title, given their spectacular pitching, but it isn't all that surprising. Amazingly, of the 71 players in the National League who qualify for the batting title (3.1 plate appearances per team's game), only one is on the Giants. To add insult to injury, that player, Aubrey Huff, has been the second worse player in the National League by WAR, being below average batting, fielding, and running the bases. It is a minor miracle that they're 6 games over .500 right now, but when you have three 4+ WAR pitchers on your roster, that goes a long way.
As a crazy aside, only the Phillies have a better top three in the game. Roy Halladay, Cole Hamels, and Cliff Lee are currently 1, 3, and 4 in the NL in WAR combining for a preposterous 17.7 wins above replacement. The Giants trio, while excellent, lags well behind at 13.4 WAR.