one problem I can think of. lets say you load 1000 rounds and didn't run the brass though sizing die. you get a new gun and it might be possible none of it will chamber. I guess im a simple sort of person and the sizing die is in the box for a reason. I just do it like I have for years and the brass lasts as long as it will last.
357 mag, 38spl, 9mm brass is cheap and very easy to come by.
there are a couple calibers I might try to get away with such a feat as not resizing brass. 25-20 is one. its very unlikely ill ever get another gun in 25-20. the brass is very thin and fragile and its pretty much impossible to find.