Staying put on the primary purpose

 The parable of the good seed and the bad seed, calls our attention to focus on the purpose and not be distracted or settle for something less (negating evil) than primarily intended (doing good).

Weeds are a nightmare for any gardner. They are those plants which have mastered every art of survival except that of growing in straight lines. Moreover they never can be got rid of. However the farmer in the parable chooses to stay focussed on growing wheat and not to focus on the weeds. He chooses to stay a farmer rather than turn detective hunting down the culprit who showed the darnel; or shift his attention to the weeds, at the cost of the wheat. 

The farmer stays put on doing good (growing useful wheat) rather than eliminating evil (weeding out the darnel). The former indirectly achieving the latter, while the latter does not always or necessarily result in the former.