Howdy Craigt
Maybe a holster maker will get on and state the purpose of the open bottom of the holster. My understanding has been the opposite of your thinking - to actulally keep dirt, lint, water, and oils out.
By being open, these debris are not "trapped", and do not accumulate. And in some instances believe it or not, some guys do no maintenance whatsoever, which can lead to a "wad" of crud lodged in the barrel.
Some of my favorite pocket holsters are Meco stitched only, no rivets, Graham, Ahern Pocket Natural , and a few I can not remember.
The reason I do not like rivets is two fold. One, if it pops, then you gotta get it repaired.
Two , I am a purist, and Love leather and fine craftsmanship. In my opinion, the leathermaker just took a short cut, the easy way out by using rivets, and they just look plain cheap!
In my opinion, it looks like the beginner uses rivets, the craftsman stitches.
Just my opinion.
Bueno Suerte