It looks as if you have a very good holster there, and as you say, it has character with that used look.
As for muzzle plugs, or end plugs, I was never too fond of them for sporting use as they seem to collect pine needles, trash and stuff that an open end would let fall through.
I have never be too enamored with plugs, usually just round the holster off instead.
However, I imagine one good reason would be that it would help maintain the roundish shape of the holster. Also, Jordan mentioned that it also acted as a stop for the revolver, insuring against sticking in the holster. But like I said, if I get to the point where I think that it is essential, I can add one without too much trouble.