Peter Holland: 
  Someone [promoted up the order] to increase the 
  rate of scoring. Often these players aren't expected 
  to be there for too long, [just there to score some 
  quick runs and  relieve the  pressure.] The term comes 
  from Baseball.

John Charnock: 
  The batting side [...] can afford to lose a pinchhitter 
  without putting them in too much trouble.

  As Micheal Holdings was probably the first to  point
  out, a pinch-hitter he has a role to play in test  matches 
  too. The 30-60 rapid runs that one expects from him,  can 
  turn the course of a low-scoring,  tension-ridden test match.

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