Over the past five seasons, Cody Ross has played 590 games in the National League, getting to know the National League pitchers very well. Jose Guillen, in contrast, has played just 85 games in the National League during that span.
Brian Sabean is a man who values experience, which explains why he's filled his team with Rowand, Renteria, Burrell, Huff, and of course, Jose Guillen. But in terms of league experience, Cody Ross is clearly more qualified, having gotten to know the National League (parks, pitchers, and other subtle nuances) very well.
Not to mention that Guillen didn't play well his last time in the National League: In 2006 with the Washington Nationals, he batted just .216 in 69 games.