On one hand it's not the safest practice and nowadays isn't allowed in CAS, on the other hand that's what you get with any military weapon with a free-floating firing pin when you chamber a round, and I don't see people throwing away their SKS rifles and the like which are designed to do exactly that every time. It won't go off on it's own from just touching the primer. Some force is needed.
However, if the hammer on a revolver snags on something, or if you hit it vigorously or you drop it, well... not the best of ideas. Statistically, if you have to do everything perfectly, a mistake will happen to someone, somewhere. Honestly, I don't know how they dealt with it back in the day of single action revolvers. I know it was a known problem even in the percussion revolver era (so in the percussion days, you had milled notches between nipples in the Remington or pins between nipples in the Colts, although they used more sensitive caps then we do today), but I really don't know how they carried their guns in the old days.