I suppose when you put them in they just reduce the amount of travel by blocking off the travel that is taken up by the spacer? So the only thing you are really doing is stopping the fork diving so much changing the geometry too much?
For it to work on air forks you need to be using your full travel and spacers do not affect the height of the forks.
I was just thinking as I have a pair of Marzocci 55s on my Alpine that are a bit too tall but putting in spacers will not change this at all me is thinking.

