Originally Posted by
guy153
Agree with everyone who said it was because frames weren't always built that well so it gave you a bit of "wiggle room". I have also seen it claimed that people used to run a fixed gear for winter training to save getting mud all over their derailleurs. Doesn't sound super-plausible to me just on the face of it.
When I started racing in the eraly 1970s, it was still very popular to use fixed gears for early season training. However, the reason had nothing to do with derailleur contamination, as you were still doing more frequent bearing overhauls and chain cleaning. The reason was to force the legs to be constantly moving, to reintroduce muscle memory for pedaling and get your spin back.
Edit: Back then, n+1 and specialized bicles were rare, except for the well off, so for most amateur racers their competition bicycle was general purpose. About the only fairly common concession, was an extra set of wheels, for training.