The main problem with monetary rewards for completing a set list of objectives is that it removes the motivation of simply being creative, and replaces it with a valuable incentive.
Think of it this way. When was the last time you were given a goal in a sandbox style game and it didn't matter how you achieved that goal, just as long as you achieved it, so you took every shortcut and tried to reduce the amount of time and effort it took as much as you could? For example, in BL's tutorial, when it says "build a staircase" so you can get through the hole in the wall, but instead of building a staircase you just jump and place bricks underneath you until you've made a little tower so that you can simply walk through the door.
This in turn reduces the quality of builds and addons, because instead of being incentivised by the desire to create, it is incentivised by the desire to get paid, and whether or not you get paid is based on a set list of objectives, in which usually have plenty of areas of exploitation for corner cutting and slacking off.
just my two cents.
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