There is a great interview with Elon Musk on youtube here. He is remarkably transparent about his reasoning. One key to his success is that he works very hard to understand motive and purpose when making decisions.
Musk makes that point that it costs about as much to fuel a rocket as it does to fuel a 747. Space launches are mostly so expensive because the vehicle is sacrificed with each launch, not because of the energy requirements for a space launch. The other big component of the expense is that rocket manufacturers charge a lot. According to Musk the value of the raw materials from which they are formed is reasonably inexpensive. Those were two hugely important realizations because they meant that space launches were not inherently expensive and therefore there is enormous potential for reducing launch costs.
By being Space X instead of Boeing the cost of launch is reduced to about 25% of conventional launches because Space X can assemble a rocket from raw materials for that much less. A re-usable vehicle, Musk predicts, would reduce launch costs by an order of magnitude.
So those are the motives and reasoning underlying the X-wing grid fins and re-entry discussed in the Slashdot summary.
