I think one fundamental error FC has done with the pledges is to focus on the whales (the few people who spend tons of money). Whales are nice and helpful, but they don't need additional care and incentive. Whales may want to make sure they are doing the right thing - which is done by "showing success" (see early bird) and by grabbing just very few spots as long as they are available. And the more little fishes (normal players who only spend about the minimum) there are, the more whales are attracted and motivated to spend money.