It seems like alot of people are wondering how a guy who is 2nd in WAR in the NL at both Baseball Reference and Fangraphs is not on the team. Here is why:
1. His value comes from non-traditional stats. He has a .389 OBP and 1.3 in defensive WAR (2nd in the league). The sabermetric revolution has only had so much influence and alot of people in baseball, including players and managers, don't know about them or don't think they are useful.
2. He has done poorly against the Giants, who are managed by the guy who picks the reserves, Bruce Bochy. The Giants have only played the Pirates 3 times this year, back in late April and McCutchen went 0 for 13. He has only a .180 career AVG with just 1 HR in 61 ABs against the Giants. So maybe that is why Bochy does not recognize how good he is.
3. He is not a standout in the traditional stats. McCutchen is not in the top 10 in HRs, RBIs or AVG or SBs. He has not won a Gold Glove and is only in his 3rd year. He has never batted .300, hit 30 HRs nor had 100 RBIs. So I guess he just does not have a great reputation (even though he should).
4. He also got off to a slow start, batting just .219 in April. Then he hit .275 in May and .347 in June (not witnessed by Bochy). So his name has not been on people's minds the whole season.
I could throw in that he plays on a team in a small market that has not had a winning season since 1992 and they have finished last in their division the last 4 years.