Getting the ball up and down from 30 yards (over a wall) isn't at all hard. They have a choice of both corners anyway. With a one (or two sometimes) man wall, you get way longer to pick up the flight of the shot, than a big wall. Other keepers will disagree I'm sure.
It's not just about being able to clearly see the shot. It's about being SET for the shot. In open play you don't know when the shot will come. From a dead ball you do. Makes a huge difference.
That doesn't really answer the debate though. Yes the wall blocked 38% of the free kicks, but maybe 95% of those 38% would have been comfortable saves for the keeper anyway? This is only really relevant for kicks 25-30yds out. No keeper would consider not having as big a wall as possible for a...
I play in goal. Any free kick over 25 yards out, I only ever have a one man wall. That's enough to stop the guy just putting his foot through it (he needs to think a BIT about directing it) but doesn't block my view of the ball.
I have let a few IN, of course, but generally right in top corners...