Having instant replay as an option now in MLB is a great way to ensure that controversal calls are decided correctly. However not being able to review other game changing plays like if a player traps or catches a ball in a decisive inning raises questions of even having it.
Can you really reverse a call that shows a ball was trapped when the umpire called it a catch? Where would the runners be awarded? Would every runner only be rewarded one base to at least get the call right and possibly not cost a team the World Series?
Some critics may argue that being able to review any play could drastically slow the game down. But if you look at the stats only 33 plays have been reviewed as of 7/6/09 since replay was approved.
The goal is to get decisive calls right and replay allows that with home runs. But does the game fully benefit from replay if it is not used to correctly make other calls?
Only the future will tell.