I can't figure out why there is a need to cut the port for the loading gate variety.
For CAS matches, it's nice to have one done that way for the times when there's a load/reload 1 on the clock. Reload 6 can be OK with a cylinder switch if you're practiced but hardly worth it for 1 round. Charlie's right, there were both types when cartridges came into being. I also like having one of each because it feels a little more realistic to have a pair of pistols that feel the same but look different.