I don't think they had to use a spacer...if and when they slid their disc on the tweaked shaft and it passed the end of the splines, the problem was resolved.
Can't speak for the others but some very, very, minor tweaking of the bottom of the end of the splines allowed my clutch disc to disengage. Also, the clutch centers vary with different models/brands and the one I used, slide past the end of the splines 3/16". This let everything disengage properly. Putting a spacer between the bell and the transmission would accomplish the same thing.
I think possibly that some folks just ground away and assumed that their disc would slide the additional 3/16" and it didn't. I may be mistaken, but to discover you're having a release problem, you've got to have your disc and pressure plate installed, and I think possibly some folks didn't want to go through the R&R process to verify the disc would slide past the end of the splines.
To me, that is the key element in a successful T5 install.
