One has to remember that part of the powder charge in the cartridge was utilized for priming the "fire lock". That's not to say that a large amount of the powder was utilized for priming, but considering "spillage" due to stress and nerves under fire, there would certainly be a question as to consistency from one load to the next.
I never did any measuring to see how consistent I was but I did utilize some paper wrapped cartridges for my Fusil de Chase one time. Personally, I much prefer (for obvious reasons) to prime a flintlock after the charge is introduced into the barrel rather than prior. However, military regulations of the time saw it differently. By priming with powder from the cartridge, t eliminated the necessity of a priming horn.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63