Well one wouldn't leave a silver service discovered in a deceased relatives belongings tarnished. You'd polish it up real good and set it in a curio cabinet or actually use it when guests come over. Old paintings are restored all the time. We do not treasure the patina on a rare painting. Eventually the Docent will have restorers come in, remove years of schmutz and the painting becomes lively and vibrant. A dull old painting that was not very eye catching suddenly becomes a prized exhibit the guide is sure to bring the tour to see.
In certain cases, the presence of patina can make it challenging to study and authenticate an antique. Patina can obscure the underlying materials or techniques used, making it difficult to determine the guns origin, age, or other hidden information. Patina can also hide structural issues in a gun that may pop up as a serious issue later on.
It just seems so absolutely artificial to me and I suspect the first collector who declared patina important was lazy and just trying to weasel out of cleaning some artifact or collection of artifacts.