I've read that the reason they used Trapdoor Springfields in some scenes in The Outlaw Josey Wales was because its safer to fire a blank cartridge than to fire a blank-loaded muzzle loader.
That's only part of the reason (although no one wants to see a ram rod go zinging across the set). The other reason is a combination of time/reliability. Generally speaking to get 60 seconds of film requires up to 45 minutes of set up (and you wondered why movies cost millions to make). This means that when the action is hot and heavy, most producers/directors can't afford to re-shoot a scene because the muzzle loader failed to fire on cue.
The use of blanks has another advantage and that is sight and sound -- they can be loaded to smoke, flash, or bang, or any combination of these three. Usually the sound is a mere pop, with the the bang added in later, in post production, when other sound effects are added to the picture.
