I believe that motoman is suggesting that the spacer keeps the silencer from hitting the sprocket if the suspension gets bottomed out. When I got my 86 the mid pipe looked to have melted a hole in the air box, but that was also a FMF slip on a stock header, a spacer like that would of prevented that problem.

Maybe with a spacer thats not a socket, like cut pipe. btw thanks for the tip on the foil, I'll have to try that when I get home.