Well, I haven't figured out what any of the sub-22 caliber rifles are for, but a fellow writing for Guns & Ammo says that the .204 is dandy for African plains game. He used 40 grain V-Max bullets to neck and lung shoot a variety of African game up to the size of a blesbok.
The smallest centerfire I've ever owned has a .264" hole in the muzzle, so maybe I'm not aware of the potential of .204" varmint bullets, but this article just seems irresponsible. I haven't hunted Africa, either, so I thought I'd ask folks that have.
As an aside, any varmint bullet that holds up well enough for other forms of hunting may very well be too tough to use for varmints. No recovered bullets are shown in the article, but I'm wondering if this bullet might not skip off the ground, or at least have large pieces skip off the ground and continue down range farther than one would want.