Blinded by competitiveness and high potential salaries, MLB veterans usually don't know when to retire. In reality, every team has a player who should discontinue his playing career immediately or following the 2013 season.
The rationale varies with the individual. In many cases, guys with limited abilities stick around past their primes to pursue the pipe dream of becoming a major league regular. You might not even realize that they are still employed.
Then there are others who have consistently survived on active rosters as reserves. They want to continue in that capacity for as long as possible (e.g. Jose Molina).
Fortunately, the former superstars have more self-awareness. Lance Berkman and Mariano Rivera, for example, understand that it's time for the next phase of their lives.