I am pretty sure that FA recommends this due to erosion of the chambers due to the shorter rounds resulting in a significant increase in the force required to eject full length 454 rounds after shooting 45 colts.

The issue as far as I remember was not due to the crud ring per se, in that even after the crud ring was scrubbed out, fired 454 rounds were still very difficult to eject. I believe that most of the issue is related to the very tight chambers that FA revolvers exhibit.