Originally posted on 05/01/2014:

First off, relievers aren't necessarily pitchers that failed @ starting or aren't good enough to be starters. Relievers as a whole have a combined era lower than starters. They also strikout more batters per 9.

Many rasons for bullpens being in OF. Injury being one of the main ones. Another main one is $$. in the old days ballparks were designed with dugouts big enough to accommodate an entire team including relief pitchers. The new ballparks have smaller dugouts & move relievers to the pen area so they can sneak in a few extra top $$ seats. Also helps managers/coaches to communicate with & find position players in the dugout, less commotion ect. makes it easier to manage.
It is also well known pitching staffs (relievers) are a closer unit & allows them to be together in OF pens & grow as a group & hopefully perform better by learn from each other. The pen allows the pitching coache(s) an area to work on things during the game as well which would be tougher if everybody were in the dugout. The pen also gives the relievers a place to store their belongings/equipment rather than cluttering the dugout & gives them an area to stretch & get loose an inning or two before they are likely to be called & allows coaches to be hands on during warm ups. The pen also isolates them during pre-game warm-UP's so they can prepare & concentrate w/o worrying about batting practice balls being lasered off their foreheads
Many reasons for bullpens being in the OF & most of them are good reasons~