Nickel brass will get harder then standard brass cases. If it hasn't been worked for awhile, it can take a lot more muscle to size a nickel rifle case then a standard brass one.
If you shoot your brass often enough and live in a low humidity environment, there is not much need for it. However I do use it in some pistols, especially if the cases may sit in leather loops for a good while.
I also like them as a visual aid, in the even that you have two very similar cartridge that can cause issues if chambered in the wrong firearm. As an example, a 6x45 can fit in a 5.56 chamber and is certain to cause problems if the trigger gets pulled on it....so if I do get a 6x45 upper, I'm likely to only load nickel brass for it...
