That's not really the case. Run production and run prevention are equally important. The thing is, at the major league level, the vast majority of run prevention occurs on the mound, as the difference between the best and worst major league fielders at a given position is relatively small, especially when compared to the difference between the best and worst hitters at the same position. So, when choosing a player, defense should be given some consideration, but much more consideration should be given to his offensive skill set, as that's where he's much more likely to be beneficial, with very rare exception.