Why does this matter?  Because muscle strength imbalances are among the leading causes of sports injuries.  For example, pulled hamstrings (back of upper leg) are often the result of quadriceps muscles (front of upper leg) that are stronger due to over-training. 

     It's more "fun" for athletes to train those quads, as they look more impressive than even well developed hams.  And therein lies a big problem:

TRAINING FOR THE WRONG REASONS 

     Playing baseball and softball requires the use of every muscle in the body.  Clearly it is to your advantage and will help your game to be STRONG and FLEXIBLE.

     Notice that I use those terms together - you need both of these attributes.

     As a ball player, your ultimate goal is POWER, the ability to do explosive work. Strength & flexibility are necessary components of power.

     So your lifting should be oriented to the acquisition of power - NOT BUILDING "COSMETIC" muscle. Leave the 18" arms and big bench press to your gym and football buddies - at least until your
playing days are over.

More specifically,

WORK EACH OPPOSING MUSCLE AND MUSCLE GROUP AS EVENLY AS POSSIBLE

     For example, do as many sets and reps of an exercise (as nearly as possible) for a muscle on one side of a joint as the other. Lifts such as biceps curls should be balanced by an equal amount of 
triceps work.  Chest (pectoral) work should be balanced by upper back and rear shoulder work (two good lifts to offset your bench work are bent-over rows and bent-over laterals.

     Don't forget quads/hams, abdominals/hips/ lower back, front shoulders/rear shoulders, and forearm flexors/extensors.