Don't know how common it is but I had a 3 HP Johnson that I picked up wouldn't run good (missed a lot). The owner had gone through and replaced the points and coils and condenser but still couldn't get it to keep running. Did all the usual stuff and couldn't find the problem. Finally took the ignition leads off of the coils and pulled them out of the motor. On checking them out I found both of them with the center core wire broken right where they passed through the guide loop between the coil and the spark plugs. Seems that all the years of moving the throttle slider had flexed them enough to fatigue the core to failure. The old insulation was breaking down and allowing the spark to jump to the loop instead of going to the plugs.
Like I said, don't know how common it is but it might be something to keep in mind with all the old motors that have throttle sliders that move the spark advance also.
One reason I change the plug wire every time I do a full tune up with new coils. I know a lot of guys on here like the old original wire, I don't buy it. I toss it, 60 year old wire is 60 year old wire.
Can't say that I've seen a lot of them actually broken, but arcing through the insulation where clamped is a pretty common problem. Usually just an almost invisible pin-hole that the spark has been arcing through.