You are correct. The straighter arm recovery is getting popular among sprinters. If the arm falls from a greater height it has more downward energy. Some believe that if the hand is positioned well (about 45 degrees angle) the falling into the catch can also cause some propulsion. It is interesting to note that some of the best coaches in history, Eddie Reese of Texas for one, does not agree with this. He teaches the high elbow low hand recovery at all speeds. Also interesting, Nathan enters thumb down which seems to eliminate this advantage and he is the fastest sprinter in our country right now. There is some detail in this concept that I don't get yet.
In general, leaving the hand lower allows more relaxation and therefore less energy use. It allows a quieter entry which conserves energy. These energy conservations are why you see this more in distance swimmers.
As always, everything in swimming is a balance and there are always exceptions. Sprinters can use a lot of energy for small gains in speed because they only need to maintain for 18 seconds.