If you only enter the MLAGB competitions, they are all timed to account for only one cylinder.
If you shoot Timed & Precision and Multi-Target (PP1 and SPB as was) or America Match or practically any other competitions then an extra cylinder (or 2 if you can afford it) is useful as you can complete a stage of ten or twelve rounds, retire to the reloading area while the next detail shoots and then swap over again. This gets maximum utilization of the range time available.
Another alternative (at a higher cost) is nitro muzzle loaders (either converted LBP or specially made). These take as long to load as muzzle loaders but generally are more indoor range friendly as they don't produce as much smoke. They are listed as a muzzle loader on your ticket so you don't have to specify Nitro versus black or substitute.
Finding the right caps (number) is essential as BP revolvers can be very unreliable if you use the wrong ones, that's why I sold mine. Bang...click...bang...bang...click...bang etc really lowers your T&P scores.