I think it's daft. Low free kicks tend to work less than conventional ones from my observation, so all you're doing is forcing the taker to take a more advantageous approach. I think there's probably an argument that the best thing is to just not have a wall at all - I think a lot of scored free kicks would be saved if the goalie could see the strike all the way rather than having his view impaired initially. Like so much of football the wall seems to be built on received wisdom rather than any proper analysis.