To get a feel for how important it is to have depth beyond the 4th OF, you really just need to have been paying attention to this team.
2015
pre-season Top 4 = Bautista, Pillar, Pompey, Saunders
time in OF for Pennington, Tolleson, Valencia, Carrera, Goins, Diaz, and Colabello = 182 games, 1118.2 innings
* team had to trade for Revere mid-season
2014
pre-season Top 4 = Bautista, Cabrera, Rasmus, Sierra
time in OF for Encarnacion, Mayberry, Reimold, Diaz, Tolleson, Gillespie, Gose, Mastroianni, Glenn, Pillar, Pompey = 202 games, 1247.33 innings
2013
pre-season top-4 = Bautista, Cabrera, Rasmus, Davis
time in OF for Bonifacio, Gose, Sierra, Pillar = 150 games, 1037.32 innings
We're talking about over 1000 innings per season being played by guys who weren't in the opening day 4-man OF rotation. Someone gets hurt, and the carousel of replacement-level garbage revs up. As things stand now, those innings are going to Junior Lake and Ezequiel Carrera. There is going to be plenty of playing time for Austin Jackson this year.