Action for reward is a stock-in-trade with all MMOs. Players want to be rewarded for their time spent but it's a conundrum for developers. See how the issue is viewed and addressed in Matt Miller's latest column.
Unfortunately, just like fluid dynamics (and electricity for the most part), the playerbase will seek out and exploit the path of least resistance. What actions will take the least amount of effort and the least amount of time to achieve the same goal, and receive the same reward? Given enough time (or enough people) such paths are found in most MMOs, much to the designers chagrin.
Read more of Matt Miller's Player Behavior & Fluid Dynamics.