There has been tons written about this, there are entire messge boards just on Clint Eastwood movies. Here's one: http://www.clinteastwood.org/forums/index.php
Something I read lately, might have even been here, that Clint was involved somehow in a chainfire during the Spaghetti-Western days and made a decision not to have Percussion revolver around.
While this is a movie, real life historical records show no (or at least not enought to write about) evidence that anyone carried extra cylinders primed and capped. It's also very important to remember that revolvers were not the primary weapon of the armies, they were more often than not, considered a liability.
Except for the bands of guarillas, who were armed to the teeth with as many revolvers as they could beg, borrow, or steal and put them everywhere. Since there IS much written about this process, and none written of the carring extra cylinders...well, how much more evidence do we need?
Then lastly, Pale Rider was the only movie that I know of, where a cylinder change was promoted. This should also tell us somthing about the use of extra cylinders. Since this was the 1st time it was actually done in a movie and now we are debating "If was regularly done in war." The evidence is in-it wasn't.
But, we can still talk about it and have fun with the idea.
I have two '58's and 6 cylinders. All JUST FOR FUN.