It is interesting that some of the folks who do NOT use tumblers seem to feel that they must argue the point about whether they are NECESSARY.
Clearly, many people have loaded for many years without tumbling their brass.
Also, many people have had problems with primers that don't seat well, cases that scratched dies, cases that don't have enough grip on the bullets, and missed defects that lead to case failures on the next firing.
And, some of the people without tumblers go to a lot of extra effort to prevent some of those problems with steps like cleaning primer pockets, washing cases, wiping down cases, polishing cases for inspections, etc.
I did not personally feel the need for a tumbler until I started shooting auto-loaders that throw my brass on the ground. The steps I NEEDED to take to ensure that the grit they picked-up would not scratch my dies or guns got pretty tedious, especially considering how many more cases I use with an auto-laoder than with a revolver or bolt gun.
For grit issues, corn-cob and walnut don't really ensure grit removal. For that matter, neither do ultrasonic cleaners. But, wet tumbling with stainless steel pins seems to remove the grit nicely. And, it removes all the fouling from the inside of the cases and the primer pockets, too. So, by decapping on a cheap Lee "C" press with a universal decapping die and wet tumbling the brass, I can keep my actual reloading equipment (and area) VERY clean. And, I can inspect the brass much more easily, using less time.
The new process is so easy that I sometimes use it for revolver and bolt gun ammo, as well.
BUt, let's not confuse "clean" with "shiny." Even after tumbling with stainless steel pins, some of my brass will have powder burns on the surface. If I want to get rid of those, I need to use some chemicals. My choice is dilluted white vinegar in an ultrasonic bath for a few minutes, followed by a rinse in water with baking soda disolved in it. That makes clean brass look UNSTAINED.
But, it still isn't jewelry-level shiny. For that, I would need to dry tumble in corn cob media with polish added. However, that is the part that I never bother doing.