We expected to get a lot of home runs from the corner infield spots this year with Pedro Alvarez at third and Lyle Overbay and Garrett Jones at first, but all 3 of those players have disappointed greatly. The outfielders haven't performed terribly, but none of them have been guys you could see hitting 25+ home runs (unless you're really high on Andrew McCutchen). The Pirates have got just 33 home runs from their core of outfielders (15 from McCutchen, 13 from Jones, 3 from Tabata, 1 from Presley, 1 from Paul, and 0 from Diaz); that's as many as Jose Bautista has hit by himself. Lance Berkman has 28 home runs by himself playing right field for the Cardinals in the same division as the Pirates this year.
The future seems bright for the Pirates, but that brightness does not include power outfield bats. Starling Marte has played very well this year in Altoona and is hitting for some power (9 home runs with a .467 SLG in 102 games), and McCutchen should be a 20 home run guy for years to come with some upside for more, but as a whole this group of outfielders just doesn't have much.
All that said, I wanted to find out how important it is for a team to have powerful outfielders. I looked to the National League's biggest success stories for my answer.
I found the last 20 teams to make the playoffs in the National League and compiled their 3 main outfielders statistics. Here's the data:
The average OPS for all those players is .828; the 2009 Andrew McCutchen was the only Pirate (at any position) to have a higher OPS than that in the last 2 years. McCutchen is the only current Pirate with an OPS higher than .828 this year.
Of the 20 NL playoff teams in the last 5 years, only 3 of them have not had an outfielder with a top 10 outfield OPS. Those three teams are the 2007 Arizona Diamondbacks, the 2006 St. Louis Cardinals, and the 2006 San Diego Padres. Those are also the only 3 teams to not have a player that was in the top 25 in OPS for any position. 7 of the 20 teams that made the playoffs had a top 10 OPS player (at any position). 9 of the 20 had an outfielder with 30 or more regular season home runs. The 2010 Giants, 2010 Braves, and 2006 Cardinals are the only 3 teams there without a 20 home run guy.
So it seems pretty clear that if a team doesn't have an outfielder that can hit for a good amount of power, their playoff chances aren't great. The Pirates definitely don't have a 30 home run threat playing the outfield anywhere in their system right now. McCutchen could possibly do it, but I wouldn't bet on it.
You can't ignore the fact that guys like Tabata and Marte are young players that are still developing, and they could hit for much more power in the future then they are right now. Nobody sees 20+ homers for Tabata, but Marte could be a guy like that. I don't see him hitting 30+, but it's a longshot possibility.
Pirates fans aren't complaining about the group of outfielders they have right now. McCutchen, Presley, Tabata, and Marte have all shown some very good signs, my concern is that as much as you don't want it to be true, the Pirates are going to need some guys that can hit 25+ home runs if they want to be competitive, and I just don't see it from anyone but Alvarez right now, and that's been really ugly this year.