Is it because not a lot of blacks can count, so they think it's an "advanced " training technique? ?
I didn't start doing 21s till a few yrs ago, it was a Asian dude told me he used to do them. I've only done it a few times before, so I incorporated them into my workout. You get a good pump (as you probably already know) Phil Heath I think did them on his vid, and Melvin Anthony did as well. They are also quick, and a more bang for less time, like supersetting bis and tris.
Is is at the beginning or the end of the workout...